Enzymatic formation of zinc-protoporphyrin by rat liver and its potential effect on hepatic heme metabolism.

The mitochondrial enzyme heme synthase (ferrochelatase) catalyzes the formation of heme from ferrous iron and protoporphyrin. Using a fluorometric assay, the enzymatic formation of zinc-protoporphyrin in rat liver tissue was compared with heme formation. With sonicated liver homogenate, the rate of zinc-protoporphyrin formation was similar to that of heme when each metal was present alone in the reaction mixture. When combined in the reaction mixture, the two metals competed for the enzyme. With intact mitochondria, zinc was a better substrate, as 90% of the product was zinc-protoporphyrin in the presence of 50 microM zinc and 50 microM iron. The effect of zinc-protoporphyrin on the induction of delta-aminolevulinic acid synthase, the rate-limiting enzyme in hepatic heme biosynthesis, was also examined in the rat. Intraperitoneal injection of allylisopropylacetamide (350 mumol) induced activity of the enzyme fourfold. Concomitant administration of zinc-protoporphyrin (4 mumol) suppressed the induction by 52%, nearly as effectively as the same amount of heme. These findings indicate that the formation of zinc-protoporphyrin in liver may (a) inhibit the synthesis of heme and (b) exert negative feedback regulation on delta-aminolevulinic acid synthase. This combination would reduce the hepatic heme level.

Bovine protoporphyria: documentation of autosomal recessive inheritance and comparison with the human disease through measurement of heme synthase activity.

Protoporphyria is an autosomal dominant disease in man in which protoporphyrin accumulated because of a defect in heme synthase (ferrochelatase) activity. A disease has been described in cattle that has the same manifestations as does the human disease. We measured heme synthase activity in sonicates of cultured skin fibroblasts and whole liver homogenates from animals with protoporphyria, their unaffected parents, and normal cattle in order to examine the mode of inheritance and compare it with human protoporphyria. The mean activity (+/- SEM) in fibroblasts from the three groups was 2.0 +/- 0.4, 47 +/- 12, and 149 +/- 10 pmol heme formed/mg protein per hr, respectively, consistent with autosomal recessive inheritance. Similarly, the levels of heme synthase activity in livers of the parents were intermediate to those of normal animals and of animals with protoporphyria. When compared with normal human fibroblasts and liver, the specific activity of heme synthase in normal bovine tissue was significantly higher. These studies indicate that manifestations of protoporphyria do not occur in cattle unless the animal is homozygous for the gene defect, whereas in humans, the heterozygous condition is sufficient. This is probably because the specific activity of heme synthase in cells of heterozygous animals is not reduced to a level that significantly alters heme metabolism.