Porphyrin biosynthesis. 3. Porphyrin metabolism in experimental lead poisoning

Heme synthesis in the lead-intoxicated mouse embryo

Incorporation of 55Fe and of [14C]glycine was studied in control embryos and mothers and in those which had received lead in the diet from day 7 of pregnancy. Incorporation of Fe into heme of embryonic liver which increases markedly for controls on day 17 of pregnancy was depressed greatly and showed no such increase in lead-intoxicated embryos. These embryos were retarded in growth but had normal heme concentrations in body and liver. Incorporation of glycine into embryonic heme and proteins was not affected. Data on incorporation in the mothers are also presented. It is thought that the impaired synthesis of heme in lead-intoxicated embryos limits their body growth during the late phase of pregnancy.

[Biochemical methods for the detection of lead poisoning (author's transl)]

After a short description of sources of lead poisoning a review is given of toxicity, absorption, storage and excretion of lead. Afterwards its biochemical affects--especially the influence on the biosynthesis of heme--are treated. Methods for diagnosis of lead poisoning resulting from these affects--determination of coproporphyrine and delta-aminolevulinic acid excretion in urine, erythrocyte delta-aminolevulinic acid dehydratase activity and erythrocyte protoporphyrin--are discussed in detail.

Effect of lead on hepatic delta-aminolaevulinic acid synthetase activity in the rat: a model for drug sensitivity in intermittent acute porphyria

The hereditary hepatic porphyrias are disorders of porphyrin and haem synthesis characterized by a marked idiosyncrasy towards a variety of lipid soluble drugs. Most of these agents are inducers of the haemoprotein cytochrome P450, the terminal oxidase in drug metabolism. The primary genetic defect in intermittent acute porphyria is a partial deficiency of uroporphyrinogen I synthetase, which may result in a secondary derepression of delta-aminoaevulinic acid synthetase, the rate-limiting enzyme in the haem pathway. Analogous defects at more distant sites may explain the other hereditary hepatic porphyrias. As drug sensitivity may be related to the defect in haem synthesis, we investigated the effects of experimental partial blocks in haem synthesis produced by lead in rats. Drug effects on delta-aminolaevulinic acid synthetase, cytochrome P450, And drug metabolism were studied. Our findings indicate: a) While partial impairment of haem biosynthesis has only minor effects on delta-aminolaevulinic acid synthetase activity, it greatly enhances the sensitivity of delta-aminolaevulinic acid synthetase to induction by drugs and steroids, which when given alone, have little or no inducing effect on the enzyme. b) The experimental partial block in haem synthesis delays and impairs drug-mediated induction cytochrome P450 and drug metabolism in vitro. The findings may explain why a large number of structurally unrelated compounds with little effect on normal liver can precipitate "aucte porphyria".

Studies on porphyrin biosynthesis in lead-intoxicated rabbits

The present report deals with studies on porphyrins and porphyrinogen carboxy-lyase of red cells and urinary porphyrins from lead-intoxicated rabbits. It was shown that the free erythrocyte porphyrins are mixture of protoporphyrin 9, the main component, and minor proportions of Coproporphyrin, Uroporphyrin III and Phyriaporphyrin. Analysis of the urinary porphyrins deomonstrates the presence of Coproporphyrins III as the major component, together with 15-20% of other porphyrins: 10-14% 5-COOH, 1-2% 6-COOH, 2-3% 7-COOH porphyrin and 1-2% Uroporphyrin III. We have not been able to detect an increase of Uroporphyrin I. Assays of porphyrinogen carboxy-lyase activity in hemolysate supernatant using Uroporphyrinogen III and Phyriaporphyrinogen (Phyria'gen) III as substrates, showed the existence of a slight decrease of both decarboxylase activities, being more affected during the second stage, the Phyria'gen decarboxylation. A possible regulation mechanism responsible for the porphyrin picture is discussed.