A newly discovered human diaphorase, designated diaphorase-4, which accounts for a major part of the diaphorase activity of most tissues but does not occur in erythrocytes, is described. In contrast with other human diaphorases, it is dependent on FAD for activity after electrophoresis, inhibited by low concentrations of dicoumarol and shows a marked affinity for Cibacron Blue. The molecular weight was estimated to be 49000 +/- 1800 by gel filtration. Diaphorase-4 appears to show person-to-person quantitative variation, so that about 4% of the population lack appreciable enzyme activity, but it is not yet clear whether this variation is of genetic or non-genetic origin.

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