Protoporphyrinogen oxidation, an enzymatic step in heme and chlorophyll synthesis: Partial characterization of the reaction in plant organelles and comparison with mammalian and bacterial systems

Abstract

High rates of oxidation of protoporphyrinogen to protoporphyrin were demonstrable in etioplasts, chloroplasts, and mitochondria from young barley shoots. Much lower rates were observed in chloroplasts from older barley or mature spinach, in mitochondria from potatoes or rat liver, and in membranes from the bacteria Escherichia coli and Rhodopseudomonas spheroides. The presence of high activity in cells capable of rapid synthesis of large amounts of chlorophyll suggests a role for this activity in chlorophyll synthesis. Characteristics of the plant protoporphyrinogen-oxidizing activity were compared to the activity in rat liver mitochondria. The activity in spinach chloroplasts exhibited a pH optimum of 7, which was lower than that of the mammalian enzyme. The plant activity was more sensitive to inhibition by glutathione or excess detergent, and was more readily inactivated at room temperature. The plant activity exhibited less specificity toward porphyrinogen substrates, oxidizing mesoporphyrinogen as rapidly as protoporphyrinogen. The mammalian enzyme oxidized mesoporphyrinogen slowly, and neither system oxidized coproporphyrinogen or uroporphyrinogen. Both the plant and the mammalian activity were bound to organelle membranes, but could be extracted with detergents. In contrast, activity from membranes of the bacteria E. coli and R. spheroides was inactivated by detergent treatment. The plant extracts could be fractionated with ammonium sulfate and retained activity after dialysis or Sephadex G-25 treatment, suggesting no readily dissociable cofactor. The activity extracted from spinach chloroplasts was mostly inactivated by trypsin digestion, which was additional evidence for the protein nature of the plant activity.