Studies on Adrenal Steroid Hydroxylases: THE REACTIVITY OF TRIPHENYL PHOSPHINE WITH LABILE SULFUR IN ADRENAL IRON-SULFUR PROTEIN (ADRENODOXIN)

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Abstract The labile sulfur of adrenodoxin was attacked by a nucleophile, triphenyl phosphine. The product was triphenyl phosphine sulfide regardless of the presence of O2. The stoichiometry between labile sulfur which disappeared and triphenyl phosphine sulfide formed was found to be 1:1. The initial rate of the reaction follows second order kinetics with a rate constant of 2 to 3 x 103 m-1 min-1 at 38° (pH 8.8) under aerobic and anaerobic conditions. The product of the minor reaction was found to be pyruvate after acid hydrolysis of the protein. The amount of pyruvate formed (about 1 mole per mole of protein) is roughly equal to those of cysteine residues lost and H2S evolved. There was no significant formation of alanine and serine by this reaction. The triphenyl phosphine-treated protein has no iron, no labile sulfur, one disulfide bond, 1 dehydroalanine, and 2 cysteine residues per molecule of the protein. On the basis of these observations, it is concluded that, upon reaction of triphenyl phosphine with adrenodoxin, the labile sulfur is removed as the phosphine sulfide, and that one of the sulfur atoms of the cysteine residues is liberated as H2S and results in the dehydroalanine residue in the polypeptide chain, which is quantitatively converted into pyruvate upon acid hydrolysis. This phenomenon is in contrast with the acid treatment which removes the labile sulfur as H2S but does not produce dehydroalanine. From the results of the triphenyl phosphine reactivity of model metal-sulfur compounds, Ni(C6H5CS3)(C6H5CS2) and Fe(C6H5CS3)2(C6H5CS2),it would be likely that the iron-sulfur grouping of adrenodoxin as its oxidation-reduction center consists of Fe—S—S— linkage, because the above model compounds contain the metal—S—S— linkage, as judged by x-ray crystallography (D. Coucouvanis and S. J. Lippard, J. Amer. Chem. Soc., 90, 3281 (1968)), and they react with triphenyl phosphine to yield its sulfide.