Abstract
Four isozymes of 3.alpha.-hydroxysteroid dehydrogenase (3.alpha.-HSD) [EC 1.1.1.50] appeared in rat livers to be classified into 3 categories depending on coenzyme requirement. Two isozymes in the 1st group had affinity for both NAD and NADP. One of the other isozymes classified in the 2nd was linked with NADP to show specificity for 5.beta.-androstan-3.alpha.-ol-17-one (etiocholanolone) as the steroid substrate. An isozyme belonging to the 3rd required only NAD as cofactor. This has the same migration rate of a lactate-dehydrogenase isozyme. In the histochemical observation, the maximal activity of the enzyme was demonstrated with 5.alpha.-androstan-3.alpha.-ol-17-one (androsterone) but not with etiocholanolone as a substrate. All 3.alpha.-HSD isozymes revealed by electrophoresis showed a higher affinity for etiocholanolone than androsterone. The zymogram of 3.alpha.-HSD in the cold acetone-treated section was essentially the same as the zymogram in the intact liver. All isozymes in the section were highest in activity when etiocholanolone was used as a substrate. These findings apparently indicate that in the cold acetone-treated section the enzyme still has affinity for etiocholanolone to resist the histochemical procedure employed.

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