The Stereospecificity of the Hill Reaction with Triphosphopyridine Nucleotide.

In 1957, San Pietro and Lang (1) observed that isolated spinach chloroplasts cause a light-stimulated exchange of the hydrogen of water with the hydrogen at the p position of the nicotinamide ring of oxidized TPN and DPN. Their results showed that there are enzymes in chloroplasts which cause activation of both steric positions of the reduced pyridine nucleotides. Since these early studies, it has been established that the photoreduction of TPN by well washed grana requires addition of two protein supplements; i.e. the iron protein, ferredoxin, and the flavoprotein, variously termed transhydrogenase or ferredoxinTPN-reductase (Z-10). Almost all of the pyridine nucleotide dehydrogenase reactions thus far examined were stereospecific for the pyridine nucleotide (11, 12). The apparent exceptions have all been reactions catalyzed by impure enzyme preparations, so that the results are of doubtful validity. One might therefore anticipate that the chloroplast enzyme which is necessary and immediately responsible for addition of hydrogen to TPN should also be stereospecific for the TPN. The present paper is concerned with the determination of the stereospecificity of the photoreduction of TPN by the spinach grana system (Equation 1).