MOLECULAR-CLONING AND EXPRESSION OF AN ABUNDANT RABBIT OVARIAN PROTEIN WITH 20-ALPHA-HYDROXYSTEROID DEHYDROGENASE-ACTIVITY

An abundant 37-kDa protein, which comprises up to 30% of the soluble proteins of the ovary, has been found to have 20alpha-hydroxysteroid dehydrogenase (20alphaHSD) activity. The steroidogenic enzyme 20alphaHSD regulates the conversion of progesterone to 20alpha-hydroxyprogesterone in many mammalian species. Complimentary DNA clones encoding a unique and abundant 20alphaHSD were isolated from a mature rabbit ovary library using guinea pig antisera generated to the purified 37-kDa protein and from a 5' EcoRI fragment from the initial positive clone. A full-length cDNA clone of 1217 basepairs encoding a 323-amino acid protein with an estimated mol wt of 37 kilodaltons was obtained. Amino acid sequence data indicate a similarity to human chlordecone reductase, bovine lung prostaglandin F synthase, human aldose reductase, human aldehyde reductase, and frog lens rho-crystallin, placing rabbit ovarian 20alphaHSD in the aldo-keto reductase family of proteins. Northern blot analysis demonstrated a 1.2-kilobase mRNA in the interstitial tissue of mature rabbit ovaries and, to a lesser extent, in corpora luteal tissue. 20alphaHSD was expressed in bacteria as a recombinant protein and was shown to possess enzymatic activity, preferring NADP as a cofactor. These studies demonstrate that an abundant ovarian protein belonging to the superfamily of NADP-dependent aldo-keto reductases has 20alphaHSD activity. This is the first example of an abundant crystallin-related protein with known enzymatic activity in a tissue other than the lens.