Dehydrogenation of androsterone by purified 3-alpha-hydroxy steroid-dependent nicotinamide-adenine dinucleotide (phosphate)-transhydrogenating enzyme of rat liver.

1. An enzyme from rat liver, catalysing 3alpha-hydroxy steroid-dependent NAD(P) transhydrogenation and NAD-linked and NADP-linked dehydrogenation of 3alpha-hydroxy steroids, has been purified 100-fold by chromatography on DEAE-cellulose and calcium phosphate gel. 2. No separation of these activities into different protein fractions has been achieved. 3. The properties of the enzyme in catalysing NAD-linked and NADP-linked dehydrogenation have been compared, with androsterone as substrate. Differences were found in pH optima, affinity for coenzyme and steroid, equilibrium constants and effects of salts. 4. NAD-linked dehydrogenation is inhibited by NADPH(2) but is protected from this inhibition by chloride, which alone is itself an inhibitor. 5. The relevance of these findings to the problem of the number of enzymes involved in catalysis of 3alpha-hydroxy steroid-dependent transhydrogenation is discussed.