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The final step in heme biosynthesis is chelation of porphyrin with Fe++ catalyzed by the mitochondrial enzyme heme synthetase. We have employed a sensitive radiochemical assay for this enzyme, using 59Fe and deuteroporphyrin or protoporphyrin as substrates. In this method iron is maintained in the ferrous state, oxygen is excluded from the incubation system, and labeled heme product is extracted into ethyl acetate. This assay has been used to measure the activity of heme synthetase in homogenates of liver, obtained by needle biopsy, and in sonicates of human skin fibroblasts, cultured in vitro. In addition, activity of the first enzyme of the heme synthetic pathway, delta-aminolevulinic acid synthetase, has been measured in fibroblast lysates. Lysates of fibroblasts from eight patients with protoporphyria had activities of delta-aminolevulinic acid synthetase which did not differ significantly from those of eight normal fibroblast lines, whereas activity of heme synthetase, with either deuteroporphyrin or protoporphyrin as substrate, was markedly decreased in sonicates of skin fibroblasts from these patients, the mean being 8% of control with deuteroporphyrin and 14% with protoporphyrin as substrate. In homogenates of liver from five patients with protoporphyria, activity of heme synthetase was also significantly less than that found in six patients without prophyria, the mean being 13% of control with protoporphyrin as substrate. These results provide evidence that decreased activity of heme synthetase is the basic defect in the heme synthetic pathway in protoporphyria. This deficiency is probably responsible for protoporphyrin accumulation and hence the biochemical and clinical features observed in protoporphyria.

Heme synthetase deficiency in human protoporphyria. Demonstration of the defect in liver and cultured skin fibroblasts.