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537. Erythrocyte adaptation to naphthoquinones
BIOCHEMICAL PHARMACOLOGY
295
rate of synthesis of the latter is influenced by the plasma level of bile salts. In fact, ingestion of cholic acid depresses liver cholesterol synthesis (Beher et al. Proc. Sac. exp. Biol., N . Y . 1961, 107, 49) and inhibits the incorporation of [~4C]mevalonic acid into the steroid. [This review usefully summarizes the factors affecting the metabolism of bile acids. The ideas expressed should also be borne in mind when considering the metabolism of foreign compounds. Thus the metabolism of a substance may be complicated (and its biological activity influenced) by its enterohepatic circulation and alteration by bacterial action.]
537. Erythroeyte adaptation to naphthoquinones Harley, J. D. & Robin, Helen (1963). Adaptive mechanisms in erythrocytes exposed to naphthoquinones. Aust. J. exp. Biol. reed. Sci. 41, 281. The oxyhaemoglobin of erythrocytes from normal adults is catalytically converted to methaemoglobin (MetHb) by various naphthoquinones (NQ). The authors have examined the relative importance of the 4 factors that govern the formation and reduction of MetHb, namely (a) the catalysis by NQ of the NADPH-dependent reduction of MetHb, (b) the normal reduction of MetHb by NADH-dependent processes, (c) the acceleration of the reaction rates of the previous 2 processes by increased MetHb concentration arid (d) the slowing up of MetHb formation by native catalase activity (see Fig. 1). Haemoglobin
S NADH
NAD 4
NADP
(b)
--
(d)
/
dependent( k processes
--
Catolose
Glucose-6phosphole
(o)
dehydrogenose
I
l(cl I I
NADH--
Glucose-6phosphate
NADPH ~
~6-Ph°sph°-"4"~ gluconote
Oxidized glutathione
Glutothione reductose
~
Reduced gluto thione
Methoemoglobin
FIG. 1. Factors affecting the formation and reduction of methaemoglobin. NAD
--Nicotinamide-adenine dinucleotide
NADH--NAD in reduced form
NADP
- - N A D phosphate
NADPH--NADP in reduced form
As we had occasion to explain on p. 246 of this issue, the normal erythrocyte contains the enzyme glucose-6-phosphate dehydrogenase which catalyses the conversion of glucose-6phosphate to 6-phosphogluconate and the protons released in this reaction maintain cell glutathione and NADP in the state of reduction essential for the adequate functioning of the erythrocyte. Cells deficient in this enzyme are more sensitive to the action of 1,2-NQ, as glutathione and NADP cannot be so readily maintained in the reduced state. Since one of the main pathways for the reduction of MetHb depends on the availability of reduced NADP, MetHb formation and a decrease in the level of reduced glutathione are both favoured. Erythrocytes drawn from the umbilical cord of the newborn are characterized by high glucose-6-phosphate dehydrogenase and glutathione reductase levels, low catalase activity, lowered ability to reduce MetHb by the NADH pathway, and haemoglobin more sensitive to oxidation to MetHb than is that of adult blood. MetHb formation on exposure to
296
TOXICOLOGY
1,2-NQ would therefore appear to be favoured, and yet little difference in reaction compared with normal adult erythrocytes was observed. It was concluded that the pentose phosphate shunt can operate in foetal erythrocytes to provide a source of protons to maintain the coenzymes glutathione and N A D P in their reduced state, thus opposing the formation of MetHb. The behaviour of 1,4-NQ, 2-methyl-l,4-NQ, 2-methyl-l,4-NQ sodium bisulphite and 2-methyl-5-hydroxy-l,4-NQ in similar experiments was essentially the same as 1,2-NQ. The compound 2-methyl-3-phytyl-l,4-NQ, on the other hand, was inert. The authors suggest that the wide natural distribution of NQ as well as nitrite [whose role in relation to glutathione was discussed (Cited in F.C.T. 1964, 2, 93)] may have provided a stimulus to the development of the mammalian erythrocyte in its present metabolic state.
•
TOXICOLOGY
538. Griseofuivin in man
Rimington, C., Morgan, P. N., Nicholls, K., Everall, J. D. & Davies, R. R. (1963). Griseofulvin administration and porphyrin metabolism. Lancet ii, 318. This Journal (Cited in F.C.T. 1963, 1, 260) provided an analysis of the situation arising from the discovery that griseofulvin (I), an antimycotic agent of great clinical value, brought about serious toxic liver damage in mice ultimately producing liver cancer. The particularly striking feature was the disturbance of pyrrole pigment metabolism of liver and bone marrow. Now we have a report of a 6-month survey of blood and faecal porphyrins in patients with tinea (ringworm) undergoing treatment with I or being followed up after the cessation of a 2-yr course of therapy. Elevated levels of various faecal porphyrins were found in: 4/6 patients with tinea who had not had I; 17-21/50 patients receiving I; 10-14/41 patients after cessation of treatment. Sigrfificantly elevated levels of erythrocyte porphyrins were not found in untreated patients; they were present in 13-20/51 undergoing therapy and 5-12/38 who had ceased therapy. Conventional tests of liver function proved negative. The conclusion is that in therapeutic doses I disturbs porphyrin metabolism in mart and that the disturbance persists for some time after the drug is withdrawn. 539. Fluoride and tissue calcification
Stookey, G. K. & Muhler, J. C. (1963). Relationship between fluoride deposition and metastatic calcification in soft tissues of rat and guinea pig. Proc. Soc. exp. Biol., N. Y. 113, 720. Recognition of the fact that fluoride (F) is deposited irt soft tissues as well as the bones artd teeth raises the problem of the relationship to calcium (Ca) and phosphorus (P) deposition. Using rats given redistilled water low in F, the authors have compared over a period of 30 days the effects of (a) low-F stock corn diet, (b) of the same diet supplemented with Ca, P and calciferol (in order to induce tissue calcification) and (c) of the administration of 1.0 mg F daily to animals on either diet. The same comparison was made over 25 days with male weanling guinea-pigs, using a rolled oat diet. On the stock diet there was no significant correlation between the amounts of Ca and F in liver and heart. In kidney, the F supplement was reflected in increased F levels without appreciable change in Ca or P. The diet inducing calcification brought about increased F levels in tissues, even without F supplementation. With the extra F, Ca levels were unaffected but F levels rose dramatically, especially in the kidneys of both rats and guinea-pigs.
295
rate of synthesis of the latter is influenced by the plasma level of bile salts. In fact, ingestion of cholic acid depresses liver cholesterol synthesis (Beher et al. Proc. Sac. exp. Biol., N . Y . 1961, 107, 49) and inhibits the incorporation of [~4C]mevalonic acid into the steroid. [This review usefully summarizes the factors affecting the metabolism of bile acids. The ideas expressed should also be borne in mind when considering the metabolism of foreign compounds. Thus the metabolism of a substance may be complicated (and its biological activity influenced) by its enterohepatic circulation and alteration by bacterial action.]
537. Erythroeyte adaptation to naphthoquinones Harley, J. D. & Robin, Helen (1963). Adaptive mechanisms in erythrocytes exposed to naphthoquinones. Aust. J. exp. Biol. reed. Sci. 41, 281. The oxyhaemoglobin of erythrocytes from normal adults is catalytically converted to methaemoglobin (MetHb) by various naphthoquinones (NQ). The authors have examined the relative importance of the 4 factors that govern the formation and reduction of MetHb, namely (a) the catalysis by NQ of the NADPH-dependent reduction of MetHb, (b) the normal reduction of MetHb by NADH-dependent processes, (c) the acceleration of the reaction rates of the previous 2 processes by increased MetHb concentration arid (d) the slowing up of MetHb formation by native catalase activity (see Fig. 1). Haemoglobin
S NADH
NAD 4
NADP
(b)
--
(d)
/
dependent( k processes
--
Catolose
Glucose-6phosphole
(o)
dehydrogenose
I
l(cl I I
NADH--
Glucose-6phosphate
NADPH ~
~6-Ph°sph°-"4"~ gluconote
Oxidized glutathione
Glutothione reductose
~
Reduced gluto thione
Methoemoglobin
FIG. 1. Factors affecting the formation and reduction of methaemoglobin. NAD
--Nicotinamide-adenine dinucleotide
NADH--NAD in reduced form
NADP
- - N A D phosphate
NADPH--NADP in reduced form
As we had occasion to explain on p. 246 of this issue, the normal erythrocyte contains the enzyme glucose-6-phosphate dehydrogenase which catalyses the conversion of glucose-6phosphate to 6-phosphogluconate and the protons released in this reaction maintain cell glutathione and NADP in the state of reduction essential for the adequate functioning of the erythrocyte. Cells deficient in this enzyme are more sensitive to the action of 1,2-NQ, as glutathione and NADP cannot be so readily maintained in the reduced state. Since one of the main pathways for the reduction of MetHb depends on the availability of reduced NADP, MetHb formation and a decrease in the level of reduced glutathione are both favoured. Erythrocytes drawn from the umbilical cord of the newborn are characterized by high glucose-6-phosphate dehydrogenase and glutathione reductase levels, low catalase activity, lowered ability to reduce MetHb by the NADH pathway, and haemoglobin more sensitive to oxidation to MetHb than is that of adult blood. MetHb formation on exposure to
296
TOXICOLOGY
1,2-NQ would therefore appear to be favoured, and yet little difference in reaction compared with normal adult erythrocytes was observed. It was concluded that the pentose phosphate shunt can operate in foetal erythrocytes to provide a source of protons to maintain the coenzymes glutathione and N A D P in their reduced state, thus opposing the formation of MetHb. The behaviour of 1,4-NQ, 2-methyl-l,4-NQ, 2-methyl-l,4-NQ sodium bisulphite and 2-methyl-5-hydroxy-l,4-NQ in similar experiments was essentially the same as 1,2-NQ. The compound 2-methyl-3-phytyl-l,4-NQ, on the other hand, was inert. The authors suggest that the wide natural distribution of NQ as well as nitrite [whose role in relation to glutathione was discussed (Cited in F.C.T. 1964, 2, 93)] may have provided a stimulus to the development of the mammalian erythrocyte in its present metabolic state.
•
TOXICOLOGY
538. Griseofuivin in man
Rimington, C., Morgan, P. N., Nicholls, K., Everall, J. D. & Davies, R. R. (1963). Griseofulvin administration and porphyrin metabolism. Lancet ii, 318. This Journal (Cited in F.C.T. 1963, 1, 260) provided an analysis of the situation arising from the discovery that griseofulvin (I), an antimycotic agent of great clinical value, brought about serious toxic liver damage in mice ultimately producing liver cancer. The particularly striking feature was the disturbance of pyrrole pigment metabolism of liver and bone marrow. Now we have a report of a 6-month survey of blood and faecal porphyrins in patients with tinea (ringworm) undergoing treatment with I or being followed up after the cessation of a 2-yr course of therapy. Elevated levels of various faecal porphyrins were found in: 4/6 patients with tinea who had not had I; 17-21/50 patients receiving I; 10-14/41 patients after cessation of treatment. Sigrfificantly elevated levels of erythrocyte porphyrins were not found in untreated patients; they were present in 13-20/51 undergoing therapy and 5-12/38 who had ceased therapy. Conventional tests of liver function proved negative. The conclusion is that in therapeutic doses I disturbs porphyrin metabolism in mart and that the disturbance persists for some time after the drug is withdrawn. 539. Fluoride and tissue calcification
Stookey, G. K. & Muhler, J. C. (1963). Relationship between fluoride deposition and metastatic calcification in soft tissues of rat and guinea pig. Proc. Soc. exp. Biol., N. Y. 113, 720. Recognition of the fact that fluoride (F) is deposited irt soft tissues as well as the bones artd teeth raises the problem of the relationship to calcium (Ca) and phosphorus (P) deposition. Using rats given redistilled water low in F, the authors have compared over a period of 30 days the effects of (a) low-F stock corn diet, (b) of the same diet supplemented with Ca, P and calciferol (in order to induce tissue calcification) and (c) of the administration of 1.0 mg F daily to animals on either diet. The same comparison was made over 25 days with male weanling guinea-pigs, using a rolled oat diet. On the stock diet there was no significant correlation between the amounts of Ca and F in liver and heart. In kidney, the F supplement was reflected in increased F levels without appreciable change in Ca or P. The diet inducing calcification brought about increased F levels in tissues, even without F supplementation. With the extra F, Ca levels were unaffected but F levels rose dramatically, especially in the kidneys of both rats and guinea-pigs.