Cytoplasmic receptor protein for etiochalanolone in chick embryo liver.

Acute intermittent porphyria is a gentic disease associated with changes in the activity of some of the hepatic enzymes in the heme biosynthetic pathway and in the activity of delta4-5alpha-steroid reductase, an enzyme involved in steroid metabolism. Embryonic chick liver has been used as a model system to study the effects of several naturally occurring 5beta steroid metabolites on delta-aminolevulinic acid synthetase, the rate-limiting enzyme in the heme pathway (Granick, S., and Kappas, A. (1967) J. Biol. Chem. 242, 4587-4593). In this study we have identified in vitro a hepatic cytoplasmic receptor which binds [3H]-etiocholanolone (a 5beta-H androgen metabolite). The steroid-receptor complex has an apparent Kd value of 3.5 times 10(-6) M at 0-4degrees; the number of binding sites per cell is 23,000. The macromolecular complex sediments at approximately 4 S in a 5 to 20% sucrose gradient and is unaffected by KCl concentrations up to 0.4 M. The steroid-receptor complex can be destroyed by heat (60degrees) or proteolytic digestion and is inhibited by sulfhydryl-blocking agents. Competition studies revealed that among the nonradioactive steroids tested, all of the primary androgens and the progestins as well as their 5alpha and 5beta metabolites block the binding of [3H]etiochalanolone. Only the glucuronide derivative of etiocholanolone, glucocorticoids and their metabolites, 17beta-estradiol, and cyproterone compete poorly for the receptor. The steroid receptor described here appears to be different from the androgen receptor isolated from rat liver and prostate.