A protein from leaves catalysing the reduction of haem–protein compounds by illuminated chloroplasts

Abstract

A protein factor from leaves, previously shown to be active in catalyzing the reduction of methemoglobin and metmyoglobin by illuminated chloroplasts, has been purified by fractionation with ammonium sulphate followed by electrophoretic separation of the active material on paper. The purified protein was found to give a single symmetrical boundary in the ultracentrifuge and in the Tiselius electrophoresis apparatus. The molecular weight of the protein, calculated from sedimentation and diffusion measurements, is 19,000. The purified protein is highly active in catalysing the photochemical reduction of metmyoglobin and also stimulates the reduction of cytochrome c, cytochrome b3 and the cytochrome components present in a particulate heart-muscle preparation. The photochemical reduction is inhibited by organic mercury compounds and this inhibition is partially reversed by cysteine. The protein does not catalyze the reduction of haem-proteins when coenzymes served as hydrogen donor. Illuminated chloroplasts were the only effective hydrogen-donating system observed. The chemical nature of the group conferring oxidation-reduction properties to the protein has not yet been determined but neither flavin nor haem could be detected.