Different characteristics of ferrochelatase in cultured fibroblasts of erythropoietic protoporphyria patients and normal controls.

Ferrochelatase activity was measured in crude extracts of fibroblasts, obtained from erythropoietic protoporphyria patients and healthy controls. The enzyme activity in erythropoietic protoporphyria fibroblasts was about 50% lower, compared to the controls. The sulfhydryl-oxidising reagent diamide inhibited the normal enzyme by about 50%, whereas ferrochelatase from erythropoietic protoporphyria fibroblasts was completely insensitive to the reagent. Pb2+ inhibits ferrochelatase activity by reacting with essential sulfhydryl groups. Low concentrations of Pb2+ inhibited the normal enzyme by 56%, but the mutant enzyme by only 8%. The photodynamic activity of bound mesoporphyrin substrate caused a biphasic inactivation of the normal enzyme. During the first 5 min of illumination a fast decrease of enzyme activity occurred to about 60% of the initial value. Experimental evidence indicates that this first phase of inactivation is caused by photooxidation of sulfhydryl groups. During further illumination inactivation continued at a much slower rate. With ferrochelatase from erythropoietic protoporphyria fibroblasts only the second, slow phase of photodynamic inactivation was observed. These observations suggest a mutation of ferrochelatase in erythropoietic protoporphyria, affecting the reactivity of sulfhydryl groups, involved in the catalytic activity of the enzyme.