- Email: [email protected]
Prooxidant and antioxidant reaction mechanisms of carotene and radical interactions with vitamins E and C
EDITORIAL
992 deficient. This observation underscores the metabolic heterogeneity in PEM, a fact that is implicit in the multifactorial origin of this condition, but that is not frequently acknowledged.’ Indeed, our observations of abnormal metabolites excretion in PEM, a reflection of other-not only biotin-vitamin deficiencies (VelBzquez, unpublished observations), points toward an extensive nutritional individuality in one of the most prevalent disorders of childhood. For obvious practical considerations, it would be impossible to apply this knowledge to the treatment of primary infant malnutrition. It may have practical implications, however, in the treatment of secondary malnutrition, such as that caused by celiac disease, where abnormalities of organic acid metabolism have recently been described.‘5 ACKNOWLEDGMENT
I thank Dr. Silvestre Frenk for his careful reading manuscript and his helpful suggestions.
5. Wolf B. Disorders of biotin metabolism. In: Striver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inherited disease, 7th ed. New York: McGraw-Hill, 1995:3151 6. Burry BJ, Sweetman L, NyWhan W. Heterogeneity of holocarboxy-
lase synthetase in patients with biotin-responsive multiple carboxylase deficiency. Am J Hum Genet 1985;37:326 7. Wolf B, Grier RE, Parker WD, Goodman SI, Allen RJ. Deficient
8.
9.
10. of the 11.
ANTONIO VELAZQUEZ, MD, PHD Unidad de GenCtica de la Nutrici6n Instituto de Investigaciones BiomCdicas, UNAM; Instituto National de Pediatria Mexico REFERENCES Waterlow JC. Profein-energy malnutrilion. London: Edward Arnold Publishers, 1992 Striver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inherited disease, 7th ed. New York: McGraw-Hill, 1995 Rosenberg LE. Vitamin-dependent genetic disease. In: McKusick VA, Claibome R, eds. Medical genetics. New York: HP Publishing, 1973:73 Rosenblatt DS, Shevell MI. Inherited disorders of cobalamin and folate absorption and metabolism, 2nd ed. Fernandes J, Saudubray JM, Van den Berghe GV, eds. Berlin: Springer-Verlag, 1995:247
EDITORIAL
COMMENTS
COMMENTS
12.
13.
biotinidase activity in late-onset multiple carboxylase deficiency. N Engl J Med 1983;308:161 Wolf B, Grier RW, Secor McVoy JR, Heard GS. Biotinidase deficiency: a novel vitamin recycling defect. J Inherit Metab Dis 1985; 8(suppl 1):53 Sweetman L, Surh L, Baker H, Peterson RM, Nyhan WL. Clinical and metabolic abnormalities in a boy with dietary deficiency of biotin. Pediatrics 1981;68:553 Mock DM, Baswell DL, Baker H, Holman RT, Sweetman L. Biotin deficiency complicating parenteral alimentation. J Pediatr 1988: 106: 762 Vellzquez A, Zamudio S, Baez A, Murguia-Corral R, Range1 Peniche B, Carrasco A. Indicators of biotin status: a study of patients on prolonged total parenteral nutrition. Eur J Clin Nutr 1990;43: 11 Velazquez A, Tercin M, Baez A, Gutierrez J, Rodriguez R. Biotin supplementation affects lymphocyte carboxylases and plasma biotin in severe protein-energy malnutrition. Am J Clin Nutr 1995;61:385 Sweetman L, Nyhan WL, Sakaati NA, et al. Organic aciduria in
neonatal multiple carboxylase deficiency. J Inherit Metab Dis 1982; 5:49 14. Wendel U, Eibler A, Sperl W, Schadewaldt P. On the differences between urinary metabolite excretion and odd-numbered fatty acid production in propionic and methylmalonic acidaemias. J Inherit Metab Dis 1995;18:584 15. Costa CG, Verhoeven NM, Kneepkens CMF, et al. Organic acid profiles resembling a P-oxidation defect in two patients with coeliac disease. J Inher Metab Dis 1996;19:177
PI1 SOS99-9007(97)00345-6
Nutrition Vol. 13, Nos. 1 l/12, 1997
Prooxidant and Antioxidant Reaction Mechanisms of Carotene and Radical Interactions with Vitamins E and C Of the 600 or so carotenoids found in nature, -40 are regularly consumed by humans. Carotenoids are commonly found in yellow, orange, and green fruit and vegetables, and naturally occurring carotenoids are added to food to enhance color. Among the carotenoids often used as colorants are pcarotene, lycopene, lutein, astaxanthin, and canthaxanthin. Such carotenoids can act as antioxidants both by quenching singlet oxygen and by scavenging free radicals, but under some
conditions prooxidant reactions may arise. This editorial concerns the free radical interactions of carotenoids, and, using recent results based on pulsed radiation techniques, we suggest molecular mechanisms for the following: 1. Synergistic antioxidant protection by carotenoids with vitamins E and C 2. The switch from antioxidant to prooxidant behavior as a function of oxygen concentration.
EDITORIAL
COMMENTS
993
THE CAROTENE-VITAMIN RADICAL
E-VITAMIN
C
INTERACTION
Early suggestions that carotenoids and a-tocopherol (TOH) can act synergistically came from the work of Palozza and Krinski’ and Mayne and Parker* with canthaxanthin. We have used nanosecond pulse radiolysis’ to generate and monitor the kinetics of the vitamin E radical (TOH’+) interaction with a range of carotenoids and observed that the radical is repaired efficiently by these carotenoids, generating the carotenoid radical cation4 TOH” + CAR +
TOH + CAR’-
In another series of experiments, our laser flash photolysis technique4 allowed us to generate the CAR’+ radicals themselves, and we find that these, in turn, are efficiently repaired by vitamin C (AscH,). Several carotenoids were studied, including p-carotene, lycopene, lutein, and zeaxanthin, and these behaved more or less in the same manner. However, astaxanthin behaved differently in that vitamin E repaired its radical cation rather than the reverse. Overall, this leads to an electron transfer scheme of the type:
suggested that P-carotene may even become prooxidant at high oxygen partial pressures. Similar work by Jorgensen and Skibsted’ in a heterogenous lipid/water system, using metmyoglobin as the radical initiator, showed an antioxidant effect for four carotenoids, including P-carotene, even in 50% oxygen, although they still observed an increase in antioxidant efficiency with decreasing oxygen tension. Tsuchihashi et al5 studied oxidation in microsomes and Kennedy and Liebler in liposomesx; both groups again found that p-carotene acts as a better antioxidant at low oxygen partial pressures, but no prooxidant effect was shown. However, two studies from Haila and co-workers9,ro on the oxidation of triglycerides have shown that B-carotene, lutein, and lycopene can all behave as prooxidants under air, but in vivo the oxygen tension will always be significantly lower than this. They also studied y-tocopherol, which acted as an antioxidant and, when present with any of the carotenoids, showed a greater inhibition of the oxidation than when alone, possibly indicating that tocopherols and carotenoids do behave synergistically. Fast reaction techniques have also been used to study radical reactions with carotenoids in an attempt to gain a better understanding of their anti-/prooxidant effects. Thus Hill et al.” showed that the CCl,O; radical reacted with carotenoids to yield an additional radical (CCl,O,-CAR)‘, which subsequently formed the carotenoid radical cation (CAR’+) CCl,O; + CAR *
The carotenoids of the macula (age-related macula degeneration is the most frequent irreversible disease causing blindness in the western world for people <65 y of age) are the hydroxy substituted zeaxanthin and lutein, and there is evidence that these OH groups tend to orientate the carotenoids across the membrane so that they are quite accessible to the water-soluble vitamin C. Since the macula is particularly exposed to oxidative damage, this orientation may well assist these carotenoids to enhance the antioxidant potential of the macular by vitamins E and C, and, in addition, we find that zeaxanthin is the most efficient of the carotenoids in repairing TOH’+. However, even the nonpolar carotenoids such as p-carotene, which are thought to be embedded deeply in the membrane, may also enhance the vitamin E/vitamin C antioxidant activity. This is because the carotene radical cation is a charged species and thus may well reorient itself much nearer to the water interface than the parent carotenoid itself. Our preliminary cellular experiments are consistent with such synergistic protection. On the other hand, Niki and co-workers’ suggest a simple additive protection based on a site-specific radical scavenging role of the nonpolar embedded carotenoid and the TOH scavenging nearer the polar interface. Whichever is the mechanism, the use of p-carotene plus vitamin C seems best as an antioxidant, particularly for smokers, who are low in vitamin C. Further work, contrasting the role of astaxanthin and the other carotenoids should help to confirm the relevance of the above scheme to the protective role of dietary carotenoids. THE ANTIOXIDANT
AND PROOXIDANT
MECHANISM
The early work of Burton and Ingold” showed that at oxygen partial pressures up to -20% (i.e., atmospheric pressure) pcarotene inhibited the oxidation of two model compounds (methyl linoleate and tetralin) from peroxyl radicals produced via thermal decomposition of AIBN (azobisisobutyronitrile) in chlorobenzene. However, they showed that at higher oxygen partial pressures the initial rate of p-carotene autoxidation increases. Hence it is a more efficient antioxidant at lower oxygen tensions. They also
[CCl,O,-CAR]’
-+ CCl,O,
+ CAR’-
We have observed a similar addition complex prior to CAR’ ’ formation for the interaction of the NO; radical with carotenoids.r2 Also Skibstedi3 has shown a similar intermediate with CHCI,/CAR systems. However, in the absence of oxygen, the carbon-centered radicals (e.g., Ccl;) gave the radical cation via direct electron transfer Ccl; + CAR +
Ccl;
+ CAR’-
Also, carbon-centered radicals react efficiently with oxygen to give the corresponding peroxyl radical (R’ + O,-+RO;), and this allows us to suggest schemes such as the following to explain a switch from anti- to prooxidant behavior with increasing oxygen concentration.i4
(2)
(1)
CAR’+ + R- + O2 [RO,--- CAR]’ RO; + CAR (3) Anti-oxidant Pro-oxidant mechanism mechanism T T Low lo?] High lo21 R’
LH LOOH + CAR -
+ CARO>
CAR
(4) (5) I Oxidation products
Of course, this scheme is not unique, and other steps such as (CAR-RO;)
+ O2 -+ ‘OO-CAR-R02
EDITORIAL
994 be included. Our main speculation is that the key prooxidant step involves the oxygen being “carried” from a peroxyl radical to the lipid via the carotenoid.
an antioxidant against lipid peroxidation. Arch Biochem Biophys 1995;323:137
could
6. Burton GW, Ingold KU. S-Carotene:
RUTH EDGE, BSc TRUSCOTT, DSc Chemistry Department Keele University Keele, Staffordshire, UK
T. GEORGE
ACKNOWLEDGMENT
10
We thank the American Institute of Cancer Research for support. 11.
REFERENCES 12. 1. Palozza P, Krinsky NI. S-Carotene and a-tocopherol are synergistic antioxidants. Arch Biochem Biophys 1992;297:184 2. Mayne ST, Parker RS. Antioxidant activity of dietary canthaxanthin. Nutr Cancer 1989;12:225 3. Bensasson RV, Land EJ, Truscott TG, eds. Excited stares and free radicals in biology and medicine. Oxford: Oxford University Press, 1993 4. Bohm F, Edge R, Land EJ, et al. Carotenoids enhance vitamin E antioxidant efficiency. J Am Chem Sot 1997;119:621 5. Tsuchihashi H, Kigoshi M, Iwatsuki M, et al. Action of p-carotene as
EDITORIAL
COMMENTS
COMMENTS
13.
14.
an unusual type of lipid antioxidant. Science 7984;224:569 Jergensen K, Skibsted LH. Carotenoid scavenging of radicals. Z Lebensm Unters Forsch 1993;196:423 Kennedy TA, Liebler DC. Peroxyl radical scavenging by B-carotene in lipid bilayers. J Biol Chem 1992;267:4658 Haila KM, Lievonen SM. Heinonen MI. Effects of lutein, lycopene, annatto, and y-tocopherol on autoxidation of triglycerides. J Agric Food Chem 1996;44:2096 Haila KM, Heinonen MI. Action of p-carotene on purified rapeseed oil during light storage. Lebensm-Wiss u-Technol 1994;27:573 Hill TJ, Land EJ, McGarvey DJ, et al. Interactions between carotenoids and the CCl,Oa radical. J Am Chem Sot 1995;117:8322 Bohm F, Tinkler JH, Truscott TG. Carotenoids protect against cell membrane damage by the nitrogen dioxide radical. Nature Med 1995; 1:98 Mortensen A, Skibsted LH. Kinetics of photobleaching of p-carotene in chloroform and formation of transient carotenoid species absorbing in the near infra-red. Free Rad Res 1996;25:355 Truscott TG. p-Carotene and disease: a suggested pro-oxidant and anti-oxidant mechanism and speculations concerning its role in cigarette smoking. J Photochem Photobiol B (Biol) 1996;35:233
PI1 SO899-9007(97)00346-S
Nutrition Vol. 13, Nos. 1 l/12, 1997
Homocysteine Metabolism in Pregnancies Complicated by Neural Tube Defects Failure of the embryonic neural tube to close normally during early pregnancy results in neural tube defects (NTDs) comprising spina bifida, anencephalus encephalocele, and iniencephalus. These severe congenital malformations of the central nervous system are a major cause of mortality and morbidity especially in pregnancy and childhood. Anencephalus and iniencephalus are universally lethal conditions with death occurring either in utero or shortly after birth. Most babies with spina bifida or encephalocele survive pregnancy but the majority have life-long disabilities and handicaps. The discovery that these devastating conditions could be largely prevented by the simple intervention of taking extra folic acid around conception].* was a major breakthrough. Periconceptional folic acid supplementation is estimated to prevent -72% of NTDs.i That folic acid prevents NTDs is beyond question. The mechanism of this effect is only now being uncovered. Folic acid could prevent NTDs by correcting a dietary deficiency or abnormal absorption of folate/folic acid or by overcoming a metabolic block. In a study of early pregnancy maternal blood levels, plasma folate and red cell folate levels, although lower in pregnancies affected by NTDs than in control pregnancies, were in the normal range for most of the affected mothers.3+ Thus the vast majority of women with an affected fetus were not folate deficient, at least in the conventional sense [red cell folate < 340 nmol/L (150 ng/mL)]. These findings were more suggestive of a problem in folate metabolism rather than simple dietary deficiency in affected
mothers. The absorption of folate/folic acid does not seem to be abnormal in mothers who have had children with NTDs.5 An absorption problem would be expected to result in low plasma folate and red cell folate levels, but this is not seen in most women with affected pregnancies .a.4 There is increasing evidence that a metabolic etiology underlies these conditions. Defects in folaterelated enzymes or processes are likely to be involved given that folic acid can prevent most NTDs.i A derangement of homocysteine metabolism (which is folate dependent) has been implicated. Homocysteine levels in early pregnancy were significantly higher in women with pregnancies affected by NTDs than in controls matched for vitamin B i2 levels, suggesting that these women had difficulty in metabolizing homocysteine.6 The case-control difference was most marked when vitamin B,a was low. In a Dutch study, both fasting levels and post-methionine load levels of homocysteine were significantly elevated in non-pregnant women who had given birth to an NTD baby compared with a control group.’ The authors suggested that this abnormality could be caused by impairment of either cystathionine synthase or homocysteine remethylation. However, cystathionine synthase activity measured in cultured skin fibroblasts taken from the methionine-intolerant women were normal. Homocysteine levels in the amniotic fluid of pregnant women carrying a fetus with an NTD were found to be higher than in control pregnancies despite similar blood concentrations of homocysteine,
992 deficient. This observation underscores the metabolic heterogeneity in PEM, a fact that is implicit in the multifactorial origin of this condition, but that is not frequently acknowledged.’ Indeed, our observations of abnormal metabolites excretion in PEM, a reflection of other-not only biotin-vitamin deficiencies (VelBzquez, unpublished observations), points toward an extensive nutritional individuality in one of the most prevalent disorders of childhood. For obvious practical considerations, it would be impossible to apply this knowledge to the treatment of primary infant malnutrition. It may have practical implications, however, in the treatment of secondary malnutrition, such as that caused by celiac disease, where abnormalities of organic acid metabolism have recently been described.‘5 ACKNOWLEDGMENT
I thank Dr. Silvestre Frenk for his careful reading manuscript and his helpful suggestions.
5. Wolf B. Disorders of biotin metabolism. In: Striver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inherited disease, 7th ed. New York: McGraw-Hill, 1995:3151 6. Burry BJ, Sweetman L, NyWhan W. Heterogeneity of holocarboxy-
lase synthetase in patients with biotin-responsive multiple carboxylase deficiency. Am J Hum Genet 1985;37:326 7. Wolf B, Grier RE, Parker WD, Goodman SI, Allen RJ. Deficient
8.
9.
10. of the 11.
ANTONIO VELAZQUEZ, MD, PHD Unidad de GenCtica de la Nutrici6n Instituto de Investigaciones BiomCdicas, UNAM; Instituto National de Pediatria Mexico REFERENCES Waterlow JC. Profein-energy malnutrilion. London: Edward Arnold Publishers, 1992 Striver CR, Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inherited disease, 7th ed. New York: McGraw-Hill, 1995 Rosenberg LE. Vitamin-dependent genetic disease. In: McKusick VA, Claibome R, eds. Medical genetics. New York: HP Publishing, 1973:73 Rosenblatt DS, Shevell MI. Inherited disorders of cobalamin and folate absorption and metabolism, 2nd ed. Fernandes J, Saudubray JM, Van den Berghe GV, eds. Berlin: Springer-Verlag, 1995:247
EDITORIAL
COMMENTS
COMMENTS
12.
13.
biotinidase activity in late-onset multiple carboxylase deficiency. N Engl J Med 1983;308:161 Wolf B, Grier RW, Secor McVoy JR, Heard GS. Biotinidase deficiency: a novel vitamin recycling defect. J Inherit Metab Dis 1985; 8(suppl 1):53 Sweetman L, Surh L, Baker H, Peterson RM, Nyhan WL. Clinical and metabolic abnormalities in a boy with dietary deficiency of biotin. Pediatrics 1981;68:553 Mock DM, Baswell DL, Baker H, Holman RT, Sweetman L. Biotin deficiency complicating parenteral alimentation. J Pediatr 1988: 106: 762 Vellzquez A, Zamudio S, Baez A, Murguia-Corral R, Range1 Peniche B, Carrasco A. Indicators of biotin status: a study of patients on prolonged total parenteral nutrition. Eur J Clin Nutr 1990;43: 11 Velazquez A, Tercin M, Baez A, Gutierrez J, Rodriguez R. Biotin supplementation affects lymphocyte carboxylases and plasma biotin in severe protein-energy malnutrition. Am J Clin Nutr 1995;61:385 Sweetman L, Nyhan WL, Sakaati NA, et al. Organic aciduria in
neonatal multiple carboxylase deficiency. J Inherit Metab Dis 1982; 5:49 14. Wendel U, Eibler A, Sperl W, Schadewaldt P. On the differences between urinary metabolite excretion and odd-numbered fatty acid production in propionic and methylmalonic acidaemias. J Inherit Metab Dis 1995;18:584 15. Costa CG, Verhoeven NM, Kneepkens CMF, et al. Organic acid profiles resembling a P-oxidation defect in two patients with coeliac disease. J Inher Metab Dis 1996;19:177
PI1 SOS99-9007(97)00345-6
Nutrition Vol. 13, Nos. 1 l/12, 1997
Prooxidant and Antioxidant Reaction Mechanisms of Carotene and Radical Interactions with Vitamins E and C Of the 600 or so carotenoids found in nature, -40 are regularly consumed by humans. Carotenoids are commonly found in yellow, orange, and green fruit and vegetables, and naturally occurring carotenoids are added to food to enhance color. Among the carotenoids often used as colorants are pcarotene, lycopene, lutein, astaxanthin, and canthaxanthin. Such carotenoids can act as antioxidants both by quenching singlet oxygen and by scavenging free radicals, but under some
conditions prooxidant reactions may arise. This editorial concerns the free radical interactions of carotenoids, and, using recent results based on pulsed radiation techniques, we suggest molecular mechanisms for the following: 1. Synergistic antioxidant protection by carotenoids with vitamins E and C 2. The switch from antioxidant to prooxidant behavior as a function of oxygen concentration.
EDITORIAL
COMMENTS
993
THE CAROTENE-VITAMIN RADICAL
E-VITAMIN
C
INTERACTION
Early suggestions that carotenoids and a-tocopherol (TOH) can act synergistically came from the work of Palozza and Krinski’ and Mayne and Parker* with canthaxanthin. We have used nanosecond pulse radiolysis’ to generate and monitor the kinetics of the vitamin E radical (TOH’+) interaction with a range of carotenoids and observed that the radical is repaired efficiently by these carotenoids, generating the carotenoid radical cation4 TOH” + CAR +
TOH + CAR’-
In another series of experiments, our laser flash photolysis technique4 allowed us to generate the CAR’+ radicals themselves, and we find that these, in turn, are efficiently repaired by vitamin C (AscH,). Several carotenoids were studied, including p-carotene, lycopene, lutein, and zeaxanthin, and these behaved more or less in the same manner. However, astaxanthin behaved differently in that vitamin E repaired its radical cation rather than the reverse. Overall, this leads to an electron transfer scheme of the type:
suggested that P-carotene may even become prooxidant at high oxygen partial pressures. Similar work by Jorgensen and Skibsted’ in a heterogenous lipid/water system, using metmyoglobin as the radical initiator, showed an antioxidant effect for four carotenoids, including P-carotene, even in 50% oxygen, although they still observed an increase in antioxidant efficiency with decreasing oxygen tension. Tsuchihashi et al5 studied oxidation in microsomes and Kennedy and Liebler in liposomesx; both groups again found that p-carotene acts as a better antioxidant at low oxygen partial pressures, but no prooxidant effect was shown. However, two studies from Haila and co-workers9,ro on the oxidation of triglycerides have shown that B-carotene, lutein, and lycopene can all behave as prooxidants under air, but in vivo the oxygen tension will always be significantly lower than this. They also studied y-tocopherol, which acted as an antioxidant and, when present with any of the carotenoids, showed a greater inhibition of the oxidation than when alone, possibly indicating that tocopherols and carotenoids do behave synergistically. Fast reaction techniques have also been used to study radical reactions with carotenoids in an attempt to gain a better understanding of their anti-/prooxidant effects. Thus Hill et al.” showed that the CCl,O; radical reacted with carotenoids to yield an additional radical (CCl,O,-CAR)‘, which subsequently formed the carotenoid radical cation (CAR’+) CCl,O; + CAR *
The carotenoids of the macula (age-related macula degeneration is the most frequent irreversible disease causing blindness in the western world for people <65 y of age) are the hydroxy substituted zeaxanthin and lutein, and there is evidence that these OH groups tend to orientate the carotenoids across the membrane so that they are quite accessible to the water-soluble vitamin C. Since the macula is particularly exposed to oxidative damage, this orientation may well assist these carotenoids to enhance the antioxidant potential of the macular by vitamins E and C, and, in addition, we find that zeaxanthin is the most efficient of the carotenoids in repairing TOH’+. However, even the nonpolar carotenoids such as p-carotene, which are thought to be embedded deeply in the membrane, may also enhance the vitamin E/vitamin C antioxidant activity. This is because the carotene radical cation is a charged species and thus may well reorient itself much nearer to the water interface than the parent carotenoid itself. Our preliminary cellular experiments are consistent with such synergistic protection. On the other hand, Niki and co-workers’ suggest a simple additive protection based on a site-specific radical scavenging role of the nonpolar embedded carotenoid and the TOH scavenging nearer the polar interface. Whichever is the mechanism, the use of p-carotene plus vitamin C seems best as an antioxidant, particularly for smokers, who are low in vitamin C. Further work, contrasting the role of astaxanthin and the other carotenoids should help to confirm the relevance of the above scheme to the protective role of dietary carotenoids. THE ANTIOXIDANT
AND PROOXIDANT
MECHANISM
The early work of Burton and Ingold” showed that at oxygen partial pressures up to -20% (i.e., atmospheric pressure) pcarotene inhibited the oxidation of two model compounds (methyl linoleate and tetralin) from peroxyl radicals produced via thermal decomposition of AIBN (azobisisobutyronitrile) in chlorobenzene. However, they showed that at higher oxygen partial pressures the initial rate of p-carotene autoxidation increases. Hence it is a more efficient antioxidant at lower oxygen tensions. They also
[CCl,O,-CAR]’
-+ CCl,O,
+ CAR’-
We have observed a similar addition complex prior to CAR’ ’ formation for the interaction of the NO; radical with carotenoids.r2 Also Skibstedi3 has shown a similar intermediate with CHCI,/CAR systems. However, in the absence of oxygen, the carbon-centered radicals (e.g., Ccl;) gave the radical cation via direct electron transfer Ccl; + CAR +
Ccl;
+ CAR’-
Also, carbon-centered radicals react efficiently with oxygen to give the corresponding peroxyl radical (R’ + O,-+RO;), and this allows us to suggest schemes such as the following to explain a switch from anti- to prooxidant behavior with increasing oxygen concentration.i4
(2)
(1)
CAR’+ + R- + O2 [RO,--- CAR]’ RO; + CAR (3) Anti-oxidant Pro-oxidant mechanism mechanism T T Low lo?] High lo21 R’
LH LOOH + CAR -
+ CARO>
CAR
(4) (5) I Oxidation products
Of course, this scheme is not unique, and other steps such as (CAR-RO;)
+ O2 -+ ‘OO-CAR-R02
EDITORIAL
994 be included. Our main speculation is that the key prooxidant step involves the oxygen being “carried” from a peroxyl radical to the lipid via the carotenoid.
an antioxidant against lipid peroxidation. Arch Biochem Biophys 1995;323:137
could
6. Burton GW, Ingold KU. S-Carotene:
RUTH EDGE, BSc TRUSCOTT, DSc Chemistry Department Keele University Keele, Staffordshire, UK
T. GEORGE
ACKNOWLEDGMENT
10
We thank the American Institute of Cancer Research for support. 11.
REFERENCES 12. 1. Palozza P, Krinsky NI. S-Carotene and a-tocopherol are synergistic antioxidants. Arch Biochem Biophys 1992;297:184 2. Mayne ST, Parker RS. Antioxidant activity of dietary canthaxanthin. Nutr Cancer 1989;12:225 3. Bensasson RV, Land EJ, Truscott TG, eds. Excited stares and free radicals in biology and medicine. Oxford: Oxford University Press, 1993 4. Bohm F, Edge R, Land EJ, et al. Carotenoids enhance vitamin E antioxidant efficiency. J Am Chem Sot 1997;119:621 5. Tsuchihashi H, Kigoshi M, Iwatsuki M, et al. Action of p-carotene as
EDITORIAL
COMMENTS
COMMENTS
13.
14.
an unusual type of lipid antioxidant. Science 7984;224:569 Jergensen K, Skibsted LH. Carotenoid scavenging of radicals. Z Lebensm Unters Forsch 1993;196:423 Kennedy TA, Liebler DC. Peroxyl radical scavenging by B-carotene in lipid bilayers. J Biol Chem 1992;267:4658 Haila KM, Lievonen SM. Heinonen MI. Effects of lutein, lycopene, annatto, and y-tocopherol on autoxidation of triglycerides. J Agric Food Chem 1996;44:2096 Haila KM, Heinonen MI. Action of p-carotene on purified rapeseed oil during light storage. Lebensm-Wiss u-Technol 1994;27:573 Hill TJ, Land EJ, McGarvey DJ, et al. Interactions between carotenoids and the CCl,Oa radical. J Am Chem Sot 1995;117:8322 Bohm F, Tinkler JH, Truscott TG. Carotenoids protect against cell membrane damage by the nitrogen dioxide radical. Nature Med 1995; 1:98 Mortensen A, Skibsted LH. Kinetics of photobleaching of p-carotene in chloroform and formation of transient carotenoid species absorbing in the near infra-red. Free Rad Res 1996;25:355 Truscott TG. p-Carotene and disease: a suggested pro-oxidant and anti-oxidant mechanism and speculations concerning its role in cigarette smoking. J Photochem Photobiol B (Biol) 1996;35:233
PI1 SO899-9007(97)00346-S
Nutrition Vol. 13, Nos. 1 l/12, 1997
Homocysteine Metabolism in Pregnancies Complicated by Neural Tube Defects Failure of the embryonic neural tube to close normally during early pregnancy results in neural tube defects (NTDs) comprising spina bifida, anencephalus encephalocele, and iniencephalus. These severe congenital malformations of the central nervous system are a major cause of mortality and morbidity especially in pregnancy and childhood. Anencephalus and iniencephalus are universally lethal conditions with death occurring either in utero or shortly after birth. Most babies with spina bifida or encephalocele survive pregnancy but the majority have life-long disabilities and handicaps. The discovery that these devastating conditions could be largely prevented by the simple intervention of taking extra folic acid around conception].* was a major breakthrough. Periconceptional folic acid supplementation is estimated to prevent -72% of NTDs.i That folic acid prevents NTDs is beyond question. The mechanism of this effect is only now being uncovered. Folic acid could prevent NTDs by correcting a dietary deficiency or abnormal absorption of folate/folic acid or by overcoming a metabolic block. In a study of early pregnancy maternal blood levels, plasma folate and red cell folate levels, although lower in pregnancies affected by NTDs than in control pregnancies, were in the normal range for most of the affected mothers.3+ Thus the vast majority of women with an affected fetus were not folate deficient, at least in the conventional sense [red cell folate < 340 nmol/L (150 ng/mL)]. These findings were more suggestive of a problem in folate metabolism rather than simple dietary deficiency in affected
mothers. The absorption of folate/folic acid does not seem to be abnormal in mothers who have had children with NTDs.5 An absorption problem would be expected to result in low plasma folate and red cell folate levels, but this is not seen in most women with affected pregnancies .a.4 There is increasing evidence that a metabolic etiology underlies these conditions. Defects in folaterelated enzymes or processes are likely to be involved given that folic acid can prevent most NTDs.i A derangement of homocysteine metabolism (which is folate dependent) has been implicated. Homocysteine levels in early pregnancy were significantly higher in women with pregnancies affected by NTDs than in controls matched for vitamin B i2 levels, suggesting that these women had difficulty in metabolizing homocysteine.6 The case-control difference was most marked when vitamin B,a was low. In a Dutch study, both fasting levels and post-methionine load levels of homocysteine were significantly elevated in non-pregnant women who had given birth to an NTD baby compared with a control group.’ The authors suggested that this abnormality could be caused by impairment of either cystathionine synthase or homocysteine remethylation. However, cystathionine synthase activity measured in cultured skin fibroblasts taken from the methionine-intolerant women were normal. Homocysteine levels in the amniotic fluid of pregnant women carrying a fetus with an NTD were found to be higher than in control pregnancies despite similar blood concentrations of homocysteine,