IDENTIFICATION OF A MAJOR PROLACTIN-REGULATED PROTEIN AS 20-ALPHA-HYDROXYSTEROID DEHYDROGENASE - COORDINATE REGULATION OF ITS ACTIVITY, PROTEIN-CONTENT, AND MESSENGER-RIBONUCLEIC-ACID EXPRESSION

We have previously reported that an abundant 37,000 mol ait protein wi th a pi of 6.15 (37K) is expressed specifically in the corpus luteum a nd is markedly inhibited by PRL. To identify the 37K, amino acid seque nce analysis of the protein was performed. The 37K; protein showed seq uence similarity with rabbit 20 alpha-hydroxysteroid dehydrogenase (20 alpha HSD), chlordecone reductase, prostaglandin synthase, and 3 alph a-hydroxysteroid dehydrogenase, which are members of the aldo-keto red uctase group of enzymes that catalyze the NADPH-dependent reduction of carbonyl compounds. Comparison of 20 alpha HSD activity with the leve l of 37K in the corpus luteum throughout pregnancy demonstrated a clos e correlation between enzyme activity and luteal levels of the protein . Both protein and enzyme activity were low early in pregnancy, reache d a nadir between days 5-19, and reappeared abruptly between days 19-2 1 of pregnancy. To establish that the enzyme activity is intrinsic to the 37K, the protein was purified from sodium dodecyl sulfate-polyacry lamide electrophoresis gels (SDS-PAGE), renatured, and assayed for 20 alpha HSD activity. The renatured protein exhibited substantial 20 alp ha HSD activity. As 20 alpha HSD is known to play a major role in the termination of pregnancy in the rat, it was of interest to examine whe ther the rapid appearance of the 37K protein at the end of pregnancy i s accompanied by the induction of 20 alpha HSD gene expres sion. North ern blot analysis using a rabbit cDNA for 20 alpha HSD indicated that the pattern of 20 alpha HSD mRNA expression in the corpus luteum close ly paralleled the ontogeny of 20 alpha HSD enzyme activity as well as 37K protein levels. Our studies demonstrated that 20 alpha HSD protein and mRNA levels are coordinately regulated, and that the profound inh ibitory effect of PRL on 20 alpha HSD activity is apparently due to in hibition of 20 alpha HSD gene expression, leading to the disappearance of the protein from the corpus luteum.