Hydrogenase Mediated Nitrite Reduction in Chlorella

The assay of the hydrogenase of glucose-grown cells of Chlorella pyrenoidosa, strain 7-11-05 by means of nitrite reduction with molecular hydrogen is described. The hydrogenase of Chlorella shows maximum activity immediately after equilibration in the hydrogen atmosphere. The hydrogenase mediated reduction of nitrite to ammonia requires the presence of CO(2). However, at pH 6.4. when the reaction proceeds optimally, there is apparently sufficient retention of metabolic CO(2) to support the reaction, which goes to completion, at near maximum rates.Reduction of nitrite in the hydrogenase system when CO(2) is present results in the uptake of 3 moles of H(2) per mole of nitrite and ammonia is the product. When CO(2) is absent or limiting, ammonia is also formed from nitrite but with the uptake of less than the stoichiometric amount of H(2). It is concluded that CO(2) is essential for the uptake of H(2), and that in the absence of CO(2) internal hydrogen donors support nitrite reduction.The possibility that CO(2) exerts a catalytic effect in all reductions mediated by hydrogenase in algae is considered, and a further hypothesis, that hydrogenase arises from that portion of the photosynthetic machinery which also shows a catalytic requirement for CO(2), is proposed.