Hemoglobin synthesis in a rabbit ribosomal system: localization of heme synthetase activity

The activity of heme synthetase (ferrochelatase), the enzyme(s) which catalyzes the formation of heme from iron and protoporphyrin IX, was studied in the various fractions of a cell-free reticulocyte system which synthesizes hemoglobin. The ribosomal fraction contained heme synthetase activity and its characteristics were similar to that described in avian erythrocytes, human and rat liver, and rabbit reticulocytes. It has a pH optimum of 7.5, required sulfhydryl reagents, was denatured by heat and was unstable on freezing. Heme synthetase is a mitochondrial enzyme. Fragments of mitochondrial membrane were identified in the ribosome fraction by both electron microscopy and the presence of cytochrome oxidase activity.