Prostaglandin‐E2 9‐Reductase from Corpus Luteum of Pseudopregnant Rabbit is a Member of the Aldo‐Keto Reductase Superfamily Featuring 20α‐Hydroxysteroid Dehydrogenase Activity

Abstract

The prostaglandin-E₂ 9-reductase (PGE₂ 9-reductase) activity in the corpus luteum of rabbits corresponds to a cytosolic, NADPH-dependent enzyme with a molecular mass of 36 kDa. This enzyme was purified from corpora lutea on day 12 of pseudopregnancy with a 266-fold enrichment. The main purification step was affinity chromatography using Red Sepharose CL-6B. The efficiency of this column was improved by elution with 1 mM NADH prior to elution of the active fractions with 1 mM NADPH. Amino acid sequence data demonstrate that the rabbit luteal PGE₂ 9-reductase has to be classified as a member of the aldo-keto reductase superfamily. The enzyme revealed a wide substrate specificity comprising the reduction of aldehydes, ketones, and quinones. Apparent kinetic constants were determined using methylglyoxal, DL-glyceraldehyde, and 9,10-phenanthrenquinone as substrates. The fully purified enzyme showed two catalytic activities of particular interest: PGE₂ 9-reductase and 20α-hydroxysteroid dehydrogenase (20α-HSD) activities. The competitive inhibition of 20α-HSD activity by PGE₂ indicates that progesterone and PGE₂ are substrates for the same enzyme. From these results, we conclude that prostaglandin and steroid metabolism are tightly linked to each other. For this reason the aldo-keto reductase could be a key enzyme in the cascade of events leading to the regression of the corpus luteum in the rabbit.