Nitrate Reduction with Molecular Hydrogen in a Reconstituted Enzymatic System

Abstract

IN recent work from this laboratory¹ it was found that flavin nucleotides, the well-known co-factors of cyclic photophosphorylation, could mediate the transfer of electrons from illuminated spinach grana to nitrate in the presence of spinach nitrate reductase. Spinach ferredoxin was not the effective electron carrier in the process. The electrons were supplied by either water or reduced indophenol dyes, and neither pyridine nucleotides nor adenosinetriphosphate were required for the reduction of nitrate. Since the dark reduction of nitrate by molecular hydrogen has been reported to occur at the cellular level in photosynthetic purple bacteria^2,3 and in the alga Ankistrodesmus braunii⁴, we investigated the mechanism of nitrate assimilation in a reconstituted enzyme system using hydrogen gas–hydrogenase as the electron donor instead of illuminated grana. In similar experiments carried out previously⁵ on the mechanism of the dark and light reduction of nitrite, it was found that the co-factor required in both cases was ferredoxin.