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The radioactive tracers H/sup 3/ and C/sup 14/ were used to study the generation, properties and metabolic implications of Scenedesmus hydrogenase. The use of tritium in the exchange reaction catalyzed by hydrogenase makes it possible to make a direct, in vivo estimate of hydrogenase activity. A quantitative and qualitative study of carbon fixation in hydrogen-adapted Scenedesmus revealed the path of carbon from CO/sub 2/ to stable organic compounds. The kinetics of hydrogenase formation show that the full complement of hydrogenase is present after about three hours. Even after only five minutes of incubation under hydrogen, detectable amounts of hydrogenase are present. The studies with dithionite showed that lowering the reduction potential of the system accelerates the formation of Scenedesmus hydrogenase. The kinetics of hydrogenase formation suggest that its formation is an autocatylitic process. Units of hydrogenase formed eralier appear to facilitate the formation of subsequent units. Adapted Scenedesmus had a higher hydrogenase activity than the Chlamydomas algae and more than representative members of two bacterial families. Scenedesmus hydrogenase exhibited the sensitivity to carbon monoxide and molecular oxygen shown by the hydrogenases found in bacterial systems. The sensitivity of Scenedesmus hydrogenase to CO and CN was a strong indication that amore » heavy metal prosthetic group is a critical component of the enzyme structure. An experiment with labeled hydrogen showed that Scenedesmus supplies electrons not protons from molecular hydrogen. In the CO/sub 2/ fixation experiments, the presence of active hydrogenase increased the quantity of carbon fixed and caused alteration in the path of fixation. The kinetic experiments showed that if an energy source is provided, CO/sub 2/ is fixed primarily along the outlines of the carbon reduction cycle. The ability of hydrogen-adapted Scenedesmus to use light to fix CO/sub 2/ when the Hill reaction is blocked by CMU was studied. The fact that photoreduction, an ATP requiring process, proceeds even in the presence of CMU indicated that hydrogenase can supply electrons to sites of the phosphorylating light reaction. It is possible that even in the absence of CMU the hydrogenase competes with the Hill reaction in supplying electrons for the phosphorylating light reaction. This would result in a lower quantum requirement for photophosphorylation during photoreduction than during photosynthesis. Clearly, hydrogenase had an effect, both quantitative and qualitative, on metabolic pathways. (auth)« less

TRACER STUDIES IN HYDROGEN-ADAPTED SCENEDESMUS USING <sup>3</sup>H AND <sup>14</sup>C