The Stereospecificity of the Hill Reaction with Diphosphopyridine Nucleotide.

Spinach grana cause little photoreduction of DPN unless they are supplemented with ferredoxin and TPN reductase. The latter flavoprotein catalyzes a reduction of DPN+ by TPNH, and has been termed a transhydrogenase on the basis of this reaction (2, 3). The results to be presented will show that the flavoprotein transhydrogenase reaction and the Hill reaction both involve hydrogen addition to the A side of the nicotinamide ring of DPN, just as for TPN. The results are not so clear-cut as those with TPN, mainly because of the presence of an interfering enzyme (or enzymes). One enzyme which complicated the studies has been identified as the lipoyl dehydrogenase of spinach leaves (4-6). Evidence will be presented that this flavoprotein has B-stereospecificity for DPN.