- Email: [email protected]
Discoloration of dental carious lesions (a review)
ARCHIVES OF
PERGAMON
Archives of Oral Biology 43 (1998) 629±632
ORAL BIOLOGY
Discoloration of dental carious lesions (a review) G.A. Kleter Department of Cariology Endodontology Pedodontology, Academic Centre for Dentistry Amsterdam (ACTA), Louwesweg 1, 1066 EA, Amsterdam, The Netherlands Accepted 7 April 1998
Abstract The discoloration of dental carious lesions is a marked feature which has received relatively little attention from dental researchers. In this short review, possible causes are considered: the formation of Maillard pigments, melanins, and lipofuscins, and the uptake of food dyes, metals, and bacterial pigments. It is concluded that the Maillard reaction between proteins and small aldehydes produced by bacteria probably accounts for the discoloration. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Caries; Pigments; Review
Dental caries is generally acknowledged as a process whereby bacterial acids destroy hard dental tissues. It is therefore not surprising that a great deal of caries research has been devoted to the de- and remineralization of enamel or dentine. Another notable but less investigated feature of the caries process is lesion discoloration. In the course of the caries process, various consecutive stages can be recognized by their colour in enamel: initial lesions appear as opaque white spots and arrested lesions as brown spots (Ripa, 1977). In root-surface caries, the generally accepted view is that an incipient lesion is slightly brown and becomes dark and hard on probing after caries arrest (Banting, 1991; Fejerskov and Nyvad, 1986). One recent report, however, describes soft, black, active lesions (Lynch and Beighton, 1994). In the super®cial white-spot enamel lesion, light is diracted dierently from that in the surrounding sound mineral, which causes the chalky white appearance. Such a physical phenomenon, however, cannot account for the brownish appearance of root dentine in incipient lesions and the dark brown and black colours of more advanced lesions in enamel and dentine lesions. There are some publications dealing speci®cally with tooth discoloration during caries, most of which date back to the 1950s and 1960s, and have been reviewed by Van Reenen (1955) and Armstrong (1964). A num-
ber of dierent hypotheses have been brought forward, but the supporting evidence does not meet present standards. In many cases, the investigators assumed that the discoloration was caused by reactions involving amino acids released from the dental matrix by proteolysis. Proteolysis of dental matrix has a key role in cavity formation according to the proteolysis±chelation theory. As the opposing acidogenesis theory gained increasing support in the 1960s, scientists probably lost interest in both proteolysis±chelation and reactions of amino acids. Investigators often simulated browning reactions on dental tissues or pure biochemical compounds under rather unnatural conditions in vitro. The resemblance between in vitro and in vivo gross features, such as elemental composition, colour, and collagen degradability, was inferred as proof that the same reaction would occur in vivo. For example, Reiss (1938) showed that isolated cariogenic bacteria induced browning of protein in the presence of the phenolic amino acid tyrosine, similar to melanin formation. Melanins are pigments that occur in hair and skin and are formed by the oxidation of tyrosine. Melanin formation is sometimes referred to as enzymatic browning. Dreizen, Armstrong and their coworkers focused on the reaction between sugar and either proteins or amino acids. Most of their work dealt with the colour,
0003-9969/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 3 - 9 9 6 9 ( 9 8 ) 0 0 0 4 8 - X
630
G. A. Kleter / Archives of Oral Biology 43 (1998) 629±632
composition, or proteolytic degradability of arti®cially modi®ed teeth, proteins, or amino acids in relation to the same properties of carious teeth (Armstrong, 1964; Dreizen et al., 1964). The sugar±protein reaction is named the Maillard reaction and encompasses a huge range of intermediates and products. It is also known as glycation or non-enzymatic browning. The Maillard reaction in human tissues is associated with the complications of diabetes and ageing, which include vascular stiening, atherosclerosis and renal insuciency. The reaction has been studied especially in the ®eld of food chemistry, where it causes characteristic changes such as the browning of bread during baking. The brown pigment is formed in a late stage of the reaction. Its exact molecular structure is still unknown. A dierent approach to solving the mechanism of browning was the puri®cation of the brown pigment formed in carious dentine. It was noted that during protein hydrolysis in concentrated acid solutions a dark precipitate formed. This precipitate was thought to contain the pigment of carious lesions. The elemental composition of the precipitate resembled that of a synthetic Maillard pigment (Dreizen and Spies, 1950). It is uncertain whether the harsh treatment by acid hydrolysis could cause arti®cial pigment formation (Armstrong, 1964). Later, a pigment was isolated from carious lesions without acid hydrolysis (Engel, 1971). The infrared spectrum of the isolate resembled that of a Maillard pigment. It showed one absorption band that was absent in sound dentine, characteristic of carbonyl groups, and marked bands for aromatic double bonds. Spectroscopy does not seem to be appropriate to distinguish melanins from Maillard pigments, as they have some properties in common. Additional indicators of the Maillard reaction would therefore be needed. Two such indicators have been observed in carious dentine: a glycosylated peptide (Armstrong, 1968) and hexitollysine (Kuboki et al., 1977). This evidence has been the most speci®c in my view, and indicates that the initial products of the Maillard reaction are formed in a carious lesion. Further research should also include indicators for the advanced stage of the reaction, in which pigment formation takes place. A number of studies present histochemical evidence for the presence of melanins in carious lesions. This evidence is insucient for two reasons. First, dierent locations of the melanins are reported: circumventing the lesion (Opdyke, 1962), diuse throughout the lesion (Ermin, 1968), and at the lesion surface (Meyer and Baume, 1966). Second, the silver stain employed in these studies cannot distinguish melanins from pigments such as lipofuscins and bile acids. Lipofuscins are formed from the oxidation of lipid molecules. Their formation from bacterial lipids in carious lesions cannot be excluded. There is only one report referring to lipid oxidation in carious material (Dirksen, 1963).
In addition to pigments, reductive substances in the dentine underlying a carious cavity reduce silver stains as well, even at an acid pH (Steinman et al., 1959). Additional stains should be used, such as Nile Blue A, which stains lipofuscins but not melanins. The above-mentioned pigments are formed from chemical reactions within a carious lesion. External pigments may represent another source of lesion stain. Kidd et al. (1990) demonstrated that carious lesions do indeed take up food dyes in vitro. To my knowledge, no data exist on the presence of food dyes in carious lesions in vivo. In addition, the lesion may take up metal ions, which form black precipitates with either bacterially formed sulphides or protein sulphide groups. Contamination of mineral by metal ions during remineralization may be another cause of discoloration. Published results on the presence of metal ions in carious lesions are contradictory. Dierent metals were found in higher amounts in carious lesions than in sound tissue: iron (Torell, 1957a,b), zinc and copper (Little and Steadman, 1966), and manganese (Bao et al., 1990). Malone et al. (1966) observed only trace amounts of metals in carious and sound dentine. This indicates that the presence of metal ions is rather circumstantial, perhaps depending on the diet. Some bacteria identi®ed in carious material are known to form pigments. For example, propionic-acid bacteria (Lee et al., 1978) and black-pigmented Porphyromonas (Shah et al., 1979) produce haem pigments. An Actinomyces strain isolated from a carious lesion formed a brown pigment (Hurst et al., 1948). Boue et al. (1987), however, found no correlation between lesion discoloration and the presence of Porphyromonas gingivalis. Neither did Bjùrndal et al. (1997) ®nd black-pigmented bacteria in every black lesion. Based on the results cited above, no ®rm conclusion is possible on the mechanism essential for carious tooth discoloration. There is, however, one stage in the caries process for which a particular mechanism seems accountable: in dentine caries, the discoloration precedes the bacteria that penetrate the demineralized dentine (Fusuyama et al., 1966). It is likely that this colour change is brought about by compounds diusing ahead of the bacteria. Because no relation has been found between lesion pigment and either pigmented bacteria or metal ions, the remaining possibilities are the Maillard reaction and the formation of either melanin or lipofuscin. Oxidation is essential for melanin and lipofuscin formation, which is therefore unlikely in this anaerobic environment. Although oxidation promotes the transformation of initial Maillard products into brown polymers, small aldehydes can react with proteins under anaerobiosis and, unlike carbohydrates such as glucose, cause browning. In addition, small aldehydes are also much more reactive than glucose,
G. A. Kleter / Archives of Oral Biology 43 (1998) 629±632
especially at pH < 7. In the anaerobic and acidic environment at the lesion front, the Maillard reaction seems therefore most likely to occur with small aldehydes derived from bacterial metabolism. The matrix would darken while the lesion front progresses with time. The outer layers, which have been subjected to the Maillard reaction longer than the lesion front, would therefore appear darker, in accordance with clinical observations. It cannot be excluded that with lesion progression, melanin and lipofuscin will add to the discoloration because the outer layers have shifted from anaerobiosis to aerobiosis, allowing oxidation to occur. Recently, the Maillard reaction in carious dentine was investigated in more detail. The content of Maillard products increases as does the Maillard-related ¯uorescence (lex 370 nm, lem 440 nm). Because no furosine was found, an established marker of the initial reaction between glucose and proteins, the reaction probably proceeds through precursors other than glucose (Kleter et al., 1998). Kuboki's ®nding that hexitollysine increases in carious dentine seems contradictory to this result (Kuboki et al., 1977). It can, however, be accounted for by the reaction between lysyl aldehyde, which increases in carious dentine (Kuboki et al., 1977), and glucosamine, instead of lysine and a hexose. The product of such a reaction would yield hexitollysine upon reduction, but no furosine after acid protein hydrolysis. In an in vitro model reaction, demineralized dentine reacted with glucose, acquiring yellow stain, after which it appeared to be less sensitive to degradation by pepsin, but not by trypsin. This indicated a change in collagen structure caused by glucose (Kleter et al., 1997). In conclusion, there are clear indications that the Maillard reaction contributes to lesion discoloration. A number of Maillard products have been elucidated recently, which are formed in tissues during ageing and diabetes. This will enable more detailed future studies on intermediates and products in carious lesions. In addition, Maillard-reaction inhibitors are receiving increased attention in diabetes research. These inhibitors include antioxidants (Ceriello et al., 1992) and aminoguanidine, which inhibits the transformation of initial to advanced Maillard products (Brownlee et al., 1986). Aminoguanidine is currently being tested clinically for the prevention of diabetic nephropathy. Inhibitors of the Maillard reaction can also help to prevent the unaesthetic discoloration of root carious lesions, which are preferably not restored.
References Armstrong, W.G., 1964. Modi®cations of the properties and compositon of the dentin matrix caused by dental caries. Adv. Oral Biol. 1, 309±332.
631
Armstrong, W.G., 1968. A method for the simultaneous separation and assays of peptides and attached carbohydrate and ¯uorescent components. Automat. Anal. Chem., Technicon Symp. 3rd, 1967 1, 295±299. Banting, D.W. 1991. Management of dental caries in the older patient. In Geriatric Dentistry. Eds Papas AS, Niessen LC, Chauncey HH. Chap. 9. Mosby Year Book, St. Louis MO, USA. pp. 141±167. Bao, M.U., Vernois, V., Deschamps, N., Revel, G., 1990. Study of physiopathological phenomena in dental enamel by neutron activation analysis. Biol. Trace Elem. Res. 26 (27), 169±176. Bjùrndal, L., Larsen, T., Thylstrup, A., 1997. A clinical and microbiological study of deep carious lesions during stepwise excavation using long treatment intervals. Caries Res. 31, 411±417. Boue, D., Armau, E., Tiraby, G., 1987. A bacteriological study of rampant caries in children. J. Dent. Res. 66, 23± 28. Brownlee, M., Vlassara, H., Kooney, A., Ulrich, P., Cerami, A., 1986. Aminoguanidine prevents diabetes-induced arterial wall protein cross-linking. Science 232, 1629±1632. Ceriello, A., Quatraro, A., Giugliano, D., 1992. New insights on non-enzymatic glycosylation may lead to therapeutic approaches for the prevention of diabetic complications. Diabet. Med. 9, 297±299. Dirksen, T.R., 1963. Lipid components of sound and carious dentin. J. Dent. Res. 42, 128±132. Dreizen, S., Spies, T.D., 1950. A note on the production of a yellow-brown pigment in the organic matrices of noncarious human teeth by oral lactobacilli. Oral Surg. Oral Med. Oral Pathol. 3, 686±691. Dreizen, S., Spirakis, C.N., Stone, R.E., 1964. In vitro studies of the chromogenic reactions between selected carbohydrate derivatives and the amino acids common to human enamel and dentine. Arch. Oral Biol. 9, 733±737. Engel, W., 1971. Zum biochemischen Ablauf der Schmelzverfaerbung beim Kariesprozess unter dem Ein¯uss verschiedener Fluorkonzentrationen. Bull. Group. Int. Rech. Sci. Stomatol. 14, 243±258. Ermin, R., 1968. Lichtmikroskopische Histologie und Histochemie der karioesen Laesionen. Bull. Group. Int. Rech. Sci. Stomatol. 11, 345±363, 431±444. Fejerskov, O., Nyvad, B. 1986. Pathology and treatment of dental caries in the aging individual. In Geriatric Dentistry. Eds Holm-Pedersen P, LoÈe H. Chap. 19. Munksgaard, Copenhagen, Denmark. pp. 238±262. Fusuyama, T., Okuse, K., Hosoda, H., 1966. Relationship between hardness, discoloration, and microbial invasion in carious dentin. J. Dent. Res. 45, 1033±1046. Hurst, V., Frisbie, H.E., Nuckolls, J., Marshall, M.S., 1948. In vitro studies of caries of the enamel in the Syrian hamster. Science 107, 42±44. Kidd, E.A.M., Joyston-Bechal, S., Smith, M.M., 1990. Staining of residual caries under freshly-packed amalgam restorations exposed to tea/chlorhexidine in vitro. Int. Dent. J. 40, 219±224. Kleter, G.A., Damen, J.J.M., Buijs, M.J., Ten Cate, J.M., 1998. Modi®cation of amino acid residues in carious dentin matrix. J. Dent. Res. 77, 488±495.
632
G. A. Kleter / Archives of Oral Biology 43 (1998) 629±632
Kleter, G.A., Damen, J.J.M., Buijs, M.J., Ten Cate, J.M., 1997. The Maillard reaction in demineralized dentin in vitro. Eur. J. Oral Sci. 105, 278±284. Kuboki, Y., Ohgushi, K., Fusuyama, T., 1977. Collagen biochemistry of the two layers of carious dentin. J. Dent. Res. 56, 1233±1237. Lee, W.L.S., Shalita, A.R., Poh-Fitzpatrick, M.B., 1978. Comparative studies of porphyrin production in Propionibacterium acnes and Propionibacterium granulosum. J. Bacteriol. 133, 811±815. Little, M.F., Steadman, L.T., 1966. Chemical and physical properties of altered and sound enamel IV. Trace element composition. Arch. Oral Biol. 11, 273±278. Lynch, E., Beighton, D., 1994. A comparison of primary root caries lesions classi®ed according to colour. Caries Res. 28, 233±239. Malone, W.F., Bell, C., Massler, M., 1966. Physicochemical characteristics of active and arrested carious lesions of dentin. J. Dent. Res. 45, 16±26. Meyer, J., Baume, L.J., 1966. Pathologie de la meÂlanodontie infantile, de l'odontoclasie et de la carie circulaire. Schw. Monatsschr. Zahnheilk. 76, 48±91.
Opdyke, D.L.J., 1962. The histochemistry of dental decay. Arch. Oral Biol. 7, 207±219. Reiss, A., 1938. Ueber Oxydasen der Karies-Erreger und ihren Ein¯uss auf die harten Zahnsubstanzen. Zahnaerztl. Rundschau 47, 2033±2041. Ripa, L.W. 1977. The nature of the carious lesion. In Oral Hygiene in Oral Health. Eds Goldberg HJV, Ripa LW. Chap. 7. Thomas, Spring®eld IL, USA. pp. 233±255. . Shah, H.N., Bonnett, R., Mateen, B., Williams, R.A.D., 1979. The porphyrin pigmentation of subspecies of Bacteroides melaninogenicus. Biochem. J. 180, 45±50. Steinman, R.R., Hewes, C.G., Woods, R.W., 1959. Histochemical analysis of lesions in incipient dental caries. J. Dent. Res. 38, 592±605. Torell, P., 1957a. Determination of iron in dental enamel. Odont. Tidskr. 65, 20±23. Torell, P., 1957b. The substance of arrested caries in relation to intact enamel. Odont. Tidskr. 65, 28±35. Van Reenen, J.F., 1955. The yellow-brown pigmentation of dental caries. A survey of the literature. J. Dent. Assoc. S. Afr. 10, 262±267.
PERGAMON
Archives of Oral Biology 43 (1998) 629±632
ORAL BIOLOGY
Discoloration of dental carious lesions (a review) G.A. Kleter Department of Cariology Endodontology Pedodontology, Academic Centre for Dentistry Amsterdam (ACTA), Louwesweg 1, 1066 EA, Amsterdam, The Netherlands Accepted 7 April 1998
Abstract The discoloration of dental carious lesions is a marked feature which has received relatively little attention from dental researchers. In this short review, possible causes are considered: the formation of Maillard pigments, melanins, and lipofuscins, and the uptake of food dyes, metals, and bacterial pigments. It is concluded that the Maillard reaction between proteins and small aldehydes produced by bacteria probably accounts for the discoloration. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Caries; Pigments; Review
Dental caries is generally acknowledged as a process whereby bacterial acids destroy hard dental tissues. It is therefore not surprising that a great deal of caries research has been devoted to the de- and remineralization of enamel or dentine. Another notable but less investigated feature of the caries process is lesion discoloration. In the course of the caries process, various consecutive stages can be recognized by their colour in enamel: initial lesions appear as opaque white spots and arrested lesions as brown spots (Ripa, 1977). In root-surface caries, the generally accepted view is that an incipient lesion is slightly brown and becomes dark and hard on probing after caries arrest (Banting, 1991; Fejerskov and Nyvad, 1986). One recent report, however, describes soft, black, active lesions (Lynch and Beighton, 1994). In the super®cial white-spot enamel lesion, light is diracted dierently from that in the surrounding sound mineral, which causes the chalky white appearance. Such a physical phenomenon, however, cannot account for the brownish appearance of root dentine in incipient lesions and the dark brown and black colours of more advanced lesions in enamel and dentine lesions. There are some publications dealing speci®cally with tooth discoloration during caries, most of which date back to the 1950s and 1960s, and have been reviewed by Van Reenen (1955) and Armstrong (1964). A num-
ber of dierent hypotheses have been brought forward, but the supporting evidence does not meet present standards. In many cases, the investigators assumed that the discoloration was caused by reactions involving amino acids released from the dental matrix by proteolysis. Proteolysis of dental matrix has a key role in cavity formation according to the proteolysis±chelation theory. As the opposing acidogenesis theory gained increasing support in the 1960s, scientists probably lost interest in both proteolysis±chelation and reactions of amino acids. Investigators often simulated browning reactions on dental tissues or pure biochemical compounds under rather unnatural conditions in vitro. The resemblance between in vitro and in vivo gross features, such as elemental composition, colour, and collagen degradability, was inferred as proof that the same reaction would occur in vivo. For example, Reiss (1938) showed that isolated cariogenic bacteria induced browning of protein in the presence of the phenolic amino acid tyrosine, similar to melanin formation. Melanins are pigments that occur in hair and skin and are formed by the oxidation of tyrosine. Melanin formation is sometimes referred to as enzymatic browning. Dreizen, Armstrong and their coworkers focused on the reaction between sugar and either proteins or amino acids. Most of their work dealt with the colour,
0003-9969/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 3 - 9 9 6 9 ( 9 8 ) 0 0 0 4 8 - X
630
G. A. Kleter / Archives of Oral Biology 43 (1998) 629±632
composition, or proteolytic degradability of arti®cially modi®ed teeth, proteins, or amino acids in relation to the same properties of carious teeth (Armstrong, 1964; Dreizen et al., 1964). The sugar±protein reaction is named the Maillard reaction and encompasses a huge range of intermediates and products. It is also known as glycation or non-enzymatic browning. The Maillard reaction in human tissues is associated with the complications of diabetes and ageing, which include vascular stiening, atherosclerosis and renal insuciency. The reaction has been studied especially in the ®eld of food chemistry, where it causes characteristic changes such as the browning of bread during baking. The brown pigment is formed in a late stage of the reaction. Its exact molecular structure is still unknown. A dierent approach to solving the mechanism of browning was the puri®cation of the brown pigment formed in carious dentine. It was noted that during protein hydrolysis in concentrated acid solutions a dark precipitate formed. This precipitate was thought to contain the pigment of carious lesions. The elemental composition of the precipitate resembled that of a synthetic Maillard pigment (Dreizen and Spies, 1950). It is uncertain whether the harsh treatment by acid hydrolysis could cause arti®cial pigment formation (Armstrong, 1964). Later, a pigment was isolated from carious lesions without acid hydrolysis (Engel, 1971). The infrared spectrum of the isolate resembled that of a Maillard pigment. It showed one absorption band that was absent in sound dentine, characteristic of carbonyl groups, and marked bands for aromatic double bonds. Spectroscopy does not seem to be appropriate to distinguish melanins from Maillard pigments, as they have some properties in common. Additional indicators of the Maillard reaction would therefore be needed. Two such indicators have been observed in carious dentine: a glycosylated peptide (Armstrong, 1968) and hexitollysine (Kuboki et al., 1977). This evidence has been the most speci®c in my view, and indicates that the initial products of the Maillard reaction are formed in a carious lesion. Further research should also include indicators for the advanced stage of the reaction, in which pigment formation takes place. A number of studies present histochemical evidence for the presence of melanins in carious lesions. This evidence is insucient for two reasons. First, dierent locations of the melanins are reported: circumventing the lesion (Opdyke, 1962), diuse throughout the lesion (Ermin, 1968), and at the lesion surface (Meyer and Baume, 1966). Second, the silver stain employed in these studies cannot distinguish melanins from pigments such as lipofuscins and bile acids. Lipofuscins are formed from the oxidation of lipid molecules. Their formation from bacterial lipids in carious lesions cannot be excluded. There is only one report referring to lipid oxidation in carious material (Dirksen, 1963).
In addition to pigments, reductive substances in the dentine underlying a carious cavity reduce silver stains as well, even at an acid pH (Steinman et al., 1959). Additional stains should be used, such as Nile Blue A, which stains lipofuscins but not melanins. The above-mentioned pigments are formed from chemical reactions within a carious lesion. External pigments may represent another source of lesion stain. Kidd et al. (1990) demonstrated that carious lesions do indeed take up food dyes in vitro. To my knowledge, no data exist on the presence of food dyes in carious lesions in vivo. In addition, the lesion may take up metal ions, which form black precipitates with either bacterially formed sulphides or protein sulphide groups. Contamination of mineral by metal ions during remineralization may be another cause of discoloration. Published results on the presence of metal ions in carious lesions are contradictory. Dierent metals were found in higher amounts in carious lesions than in sound tissue: iron (Torell, 1957a,b), zinc and copper (Little and Steadman, 1966), and manganese (Bao et al., 1990). Malone et al. (1966) observed only trace amounts of metals in carious and sound dentine. This indicates that the presence of metal ions is rather circumstantial, perhaps depending on the diet. Some bacteria identi®ed in carious material are known to form pigments. For example, propionic-acid bacteria (Lee et al., 1978) and black-pigmented Porphyromonas (Shah et al., 1979) produce haem pigments. An Actinomyces strain isolated from a carious lesion formed a brown pigment (Hurst et al., 1948). Boue et al. (1987), however, found no correlation between lesion discoloration and the presence of Porphyromonas gingivalis. Neither did Bjùrndal et al. (1997) ®nd black-pigmented bacteria in every black lesion. Based on the results cited above, no ®rm conclusion is possible on the mechanism essential for carious tooth discoloration. There is, however, one stage in the caries process for which a particular mechanism seems accountable: in dentine caries, the discoloration precedes the bacteria that penetrate the demineralized dentine (Fusuyama et al., 1966). It is likely that this colour change is brought about by compounds diusing ahead of the bacteria. Because no relation has been found between lesion pigment and either pigmented bacteria or metal ions, the remaining possibilities are the Maillard reaction and the formation of either melanin or lipofuscin. Oxidation is essential for melanin and lipofuscin formation, which is therefore unlikely in this anaerobic environment. Although oxidation promotes the transformation of initial Maillard products into brown polymers, small aldehydes can react with proteins under anaerobiosis and, unlike carbohydrates such as glucose, cause browning. In addition, small aldehydes are also much more reactive than glucose,
G. A. Kleter / Archives of Oral Biology 43 (1998) 629±632
especially at pH < 7. In the anaerobic and acidic environment at the lesion front, the Maillard reaction seems therefore most likely to occur with small aldehydes derived from bacterial metabolism. The matrix would darken while the lesion front progresses with time. The outer layers, which have been subjected to the Maillard reaction longer than the lesion front, would therefore appear darker, in accordance with clinical observations. It cannot be excluded that with lesion progression, melanin and lipofuscin will add to the discoloration because the outer layers have shifted from anaerobiosis to aerobiosis, allowing oxidation to occur. Recently, the Maillard reaction in carious dentine was investigated in more detail. The content of Maillard products increases as does the Maillard-related ¯uorescence (lex 370 nm, lem 440 nm). Because no furosine was found, an established marker of the initial reaction between glucose and proteins, the reaction probably proceeds through precursors other than glucose (Kleter et al., 1998). Kuboki's ®nding that hexitollysine increases in carious dentine seems contradictory to this result (Kuboki et al., 1977). It can, however, be accounted for by the reaction between lysyl aldehyde, which increases in carious dentine (Kuboki et al., 1977), and glucosamine, instead of lysine and a hexose. The product of such a reaction would yield hexitollysine upon reduction, but no furosine after acid protein hydrolysis. In an in vitro model reaction, demineralized dentine reacted with glucose, acquiring yellow stain, after which it appeared to be less sensitive to degradation by pepsin, but not by trypsin. This indicated a change in collagen structure caused by glucose (Kleter et al., 1997). In conclusion, there are clear indications that the Maillard reaction contributes to lesion discoloration. A number of Maillard products have been elucidated recently, which are formed in tissues during ageing and diabetes. This will enable more detailed future studies on intermediates and products in carious lesions. In addition, Maillard-reaction inhibitors are receiving increased attention in diabetes research. These inhibitors include antioxidants (Ceriello et al., 1992) and aminoguanidine, which inhibits the transformation of initial to advanced Maillard products (Brownlee et al., 1986). Aminoguanidine is currently being tested clinically for the prevention of diabetic nephropathy. Inhibitors of the Maillard reaction can also help to prevent the unaesthetic discoloration of root carious lesions, which are preferably not restored.
References Armstrong, W.G., 1964. Modi®cations of the properties and compositon of the dentin matrix caused by dental caries. Adv. Oral Biol. 1, 309±332.
631
Armstrong, W.G., 1968. A method for the simultaneous separation and assays of peptides and attached carbohydrate and ¯uorescent components. Automat. Anal. Chem., Technicon Symp. 3rd, 1967 1, 295±299. Banting, D.W. 1991. Management of dental caries in the older patient. In Geriatric Dentistry. Eds Papas AS, Niessen LC, Chauncey HH. Chap. 9. Mosby Year Book, St. Louis MO, USA. pp. 141±167. Bao, M.U., Vernois, V., Deschamps, N., Revel, G., 1990. Study of physiopathological phenomena in dental enamel by neutron activation analysis. Biol. Trace Elem. Res. 26 (27), 169±176. Bjùrndal, L., Larsen, T., Thylstrup, A., 1997. A clinical and microbiological study of deep carious lesions during stepwise excavation using long treatment intervals. Caries Res. 31, 411±417. Boue, D., Armau, E., Tiraby, G., 1987. A bacteriological study of rampant caries in children. J. Dent. Res. 66, 23± 28. Brownlee, M., Vlassara, H., Kooney, A., Ulrich, P., Cerami, A., 1986. Aminoguanidine prevents diabetes-induced arterial wall protein cross-linking. Science 232, 1629±1632. Ceriello, A., Quatraro, A., Giugliano, D., 1992. New insights on non-enzymatic glycosylation may lead to therapeutic approaches for the prevention of diabetic complications. Diabet. Med. 9, 297±299. Dirksen, T.R., 1963. Lipid components of sound and carious dentin. J. Dent. Res. 42, 128±132. Dreizen, S., Spies, T.D., 1950. A note on the production of a yellow-brown pigment in the organic matrices of noncarious human teeth by oral lactobacilli. Oral Surg. Oral Med. Oral Pathol. 3, 686±691. Dreizen, S., Spirakis, C.N., Stone, R.E., 1964. In vitro studies of the chromogenic reactions between selected carbohydrate derivatives and the amino acids common to human enamel and dentine. Arch. Oral Biol. 9, 733±737. Engel, W., 1971. Zum biochemischen Ablauf der Schmelzverfaerbung beim Kariesprozess unter dem Ein¯uss verschiedener Fluorkonzentrationen. Bull. Group. Int. Rech. Sci. Stomatol. 14, 243±258. Ermin, R., 1968. Lichtmikroskopische Histologie und Histochemie der karioesen Laesionen. Bull. Group. Int. Rech. Sci. Stomatol. 11, 345±363, 431±444. Fejerskov, O., Nyvad, B. 1986. Pathology and treatment of dental caries in the aging individual. In Geriatric Dentistry. Eds Holm-Pedersen P, LoÈe H. Chap. 19. Munksgaard, Copenhagen, Denmark. pp. 238±262. Fusuyama, T., Okuse, K., Hosoda, H., 1966. Relationship between hardness, discoloration, and microbial invasion in carious dentin. J. Dent. Res. 45, 1033±1046. Hurst, V., Frisbie, H.E., Nuckolls, J., Marshall, M.S., 1948. In vitro studies of caries of the enamel in the Syrian hamster. Science 107, 42±44. Kidd, E.A.M., Joyston-Bechal, S., Smith, M.M., 1990. Staining of residual caries under freshly-packed amalgam restorations exposed to tea/chlorhexidine in vitro. Int. Dent. J. 40, 219±224. Kleter, G.A., Damen, J.J.M., Buijs, M.J., Ten Cate, J.M., 1998. Modi®cation of amino acid residues in carious dentin matrix. J. Dent. Res. 77, 488±495.
632
G. A. Kleter / Archives of Oral Biology 43 (1998) 629±632
Kleter, G.A., Damen, J.J.M., Buijs, M.J., Ten Cate, J.M., 1997. The Maillard reaction in demineralized dentin in vitro. Eur. J. Oral Sci. 105, 278±284. Kuboki, Y., Ohgushi, K., Fusuyama, T., 1977. Collagen biochemistry of the two layers of carious dentin. J. Dent. Res. 56, 1233±1237. Lee, W.L.S., Shalita, A.R., Poh-Fitzpatrick, M.B., 1978. Comparative studies of porphyrin production in Propionibacterium acnes and Propionibacterium granulosum. J. Bacteriol. 133, 811±815. Little, M.F., Steadman, L.T., 1966. Chemical and physical properties of altered and sound enamel IV. Trace element composition. Arch. Oral Biol. 11, 273±278. Lynch, E., Beighton, D., 1994. A comparison of primary root caries lesions classi®ed according to colour. Caries Res. 28, 233±239. Malone, W.F., Bell, C., Massler, M., 1966. Physicochemical characteristics of active and arrested carious lesions of dentin. J. Dent. Res. 45, 16±26. Meyer, J., Baume, L.J., 1966. Pathologie de la meÂlanodontie infantile, de l'odontoclasie et de la carie circulaire. Schw. Monatsschr. Zahnheilk. 76, 48±91.
Opdyke, D.L.J., 1962. The histochemistry of dental decay. Arch. Oral Biol. 7, 207±219. Reiss, A., 1938. Ueber Oxydasen der Karies-Erreger und ihren Ein¯uss auf die harten Zahnsubstanzen. Zahnaerztl. Rundschau 47, 2033±2041. Ripa, L.W. 1977. The nature of the carious lesion. In Oral Hygiene in Oral Health. Eds Goldberg HJV, Ripa LW. Chap. 7. Thomas, Spring®eld IL, USA. pp. 233±255. . Shah, H.N., Bonnett, R., Mateen, B., Williams, R.A.D., 1979. The porphyrin pigmentation of subspecies of Bacteroides melaninogenicus. Biochem. J. 180, 45±50. Steinman, R.R., Hewes, C.G., Woods, R.W., 1959. Histochemical analysis of lesions in incipient dental caries. J. Dent. Res. 38, 592±605. Torell, P., 1957a. Determination of iron in dental enamel. Odont. Tidskr. 65, 20±23. Torell, P., 1957b. The substance of arrested caries in relation to intact enamel. Odont. Tidskr. 65, 28±35. Van Reenen, J.F., 1955. The yellow-brown pigmentation of dental caries. A survey of the literature. J. Dent. Assoc. S. Afr. 10, 262±267.