The steric course with respect to the reduced nicotinamide-adenine dinucleotide of the reduction of 3-oxo steroids catalysed by cortisone reductase.

Abstract

The stereochemistry of hydrogen transfer from NADH during the reduction of the 3‐oxo groups of various steroids by cortisone reductase has been investigated for a series of substrates showing interesting structural features or reaction characteristics. It was found that the 4B‐hydrogen of the NADH was transferred to the substrate in all cases whether the resulting hydroxyl group was 3α or 3β. The 4B‐hydrogen was also transferred when the same enzyme catalysed the reduction of the 20‐oxo group of pregn‐4‐ene‐3,20‐dione, so, although the enzyme can bind steroids allowing hydrogen transfer in various ways, it binds NADH in such a way that only the 4B‐hydrogen is available for catalytic transfer to any considerable extent. No reaction was detected with NADPH.The reaction rate with NADH was lowered by the presence of NAD+ in the two cases tested and, in the one case in which the reaction was studied in the reverse direction, NADH lowered the rate of reaction with NAD+. In all three cases, the apparent V values did not change significantly, whereas the apparent Km values for the reacting nucleotide changed considerably (“competitive inhibition”). This was consistent with an ordered reaction sequence in which the nucleotide bound before and left after the steroid, but these findings did not eliminate other possibilities.