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The stereospecificity of hydrogen transfer for the pyridine nucleotide has been ascertained for a number of hitherto uninvestigated dehydrogenase reactions. These determinations have been made with the help of appropriate reference enzymes, and of DPN and TPN labeled with tritium at the 4 position of the nicotinamide ring. Evidence is presented that the following enzymes use the same position (i.e., A or a) as yeast alcohol dehydrogenase: DPN-linked formic dehydrogenase from pea seeds, DPN-linked glyoxylic reductase from spinach leaves, TPN-linked glyoxylic reductase from pea seeds, TPN-linked dihydroorotic dehydrogenase from an aerobic bacterium, and TPN-linked malic enzyme from liver. The opposite stereospecificity (B or ß) is demonstrated for the DPN-linked liver dehydrogenase which oxidizes uridine diphosphoglucose to uridine diphosphoglucuronic acid.

The components of the couple NAD +-NADH interact with each other. Similar interactions were demonstrated with the components of NADP+-NADPH and (1-n-propyl nicotinamide chloride)-(l-n-propyl-l,4-dihydronicotinamide) systems. In each case, there is a nonstereospecific hydrogen exchange with no net oxidation-reduction and the formation of a reversible complex. Another colored complex is formed also, but there is no evidence that the formations of these two complexes are dependent upon each other.

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