Terminal Enzymes of Heme Biosynthesis in the Plant Plasma Membrane

Purified plasma membrane fractions from barley leaf exhibited activities of ferrochelatase and iron reductase, two of the terminal enzymes in heme biosynthesis. These activities were also present in purified barley leaf plastid and mitochondrial fractions. The plasma membrane fractions were shown to be free from contamination with plastid and mitochondrial membrane markers. Previous studies had demonstrated that barley plasma membranes exhibited activity for converting protoporphyrinogen to protoporphyrin, another late step in heme synthesis. The presence of the terminal steps of heme synthesis in the plant plasma membrane is compatible with the hypothesis that late heme precursors such as protoporphyrin or protoporphyrinogen synthesized in the chloroplast can be exported and converted to heme within the plasma membrane for subsequent incorporation into plasma membrane hemoproteins.