Some Aspects of Hydrogenase Activity and Nitrogen Fixation in Azotobacter spp and in Clostridium pasteurianum

Abstract

Hydrogenases from Azotobacter vinelandii and Clostridium pas- teurianum reduced methylene blue, ferricyanide, benzyl- and methyl-viologens when hydrogen was the donor. Methylene blue was the most effective acceptor. Hughes press preparations of either organism in tris (2-hydroxymethyl propane-l:3-diol) buffer (pH 8.0) resulted in the best extraction of the enzyme. Hydrogen-ase from C. pasteurianum was readily inactivated by traces of oxygen but this could be prevented by sodium dithionite. Pyridine nucleotides and cytochrome c. are reduced by hydrogenase, but in extracts of Azotobacter, however, the addition of iron was required to couple reduced diphosphopyridine nucleotide to cytochrome c. The mechanism appears to involve the reduction of Fe3+ to Fe2+ enzymically and the Fe2+ is oxidized by cytochrome c nonenzymi-cally. The effect of pH value and nature of buffer on this system was examined. Mo5+ did not reduce cytochrome c, with or without hydrogenase. Metal-deficiency experiments, inhibitor studies, activation of dialyzed preparations, and the results of radioactive tracer assays of purified protein fractions, showed that iron is the main metal constituent of hydrogenase. Molybdenum, however, is required for the fixation of nitrogen.