Radioimmunoassay of 5 alpha-pregnane-3,20-dione. A metabolite of placental progesterone.

A radioimmunoassay (RIA) procedure has been developed for the measurement of 5 alpha-pregnane-3,20-dione (DHP) in human plasma after ether extraction of the plasma-samples, followed by column chromatography. The antiserum was generated in rabbits with 6 alpha-carboxyethylmer-captoprogesterone-BSA conjugate. The high affinity (Ka = 3.72 X 10(9) l/mol) antiserum binds 40% of 40 picograms of tritiated 5 alpha-pregnane-3,20-dione at working dilutions of 1 : 2800. Negligible cross-reactivity of the antiserum was detected with 5 beta-pregnane-3,20-dione (1.8%). Other hydroxylated pregnanes showed minor cross-reactivity (5-40%). The cross-reacting steroids were all separated from pregnanedione by one chromatographic step. The plasma levels of 5 alpha-pregnane-3,20-dione were measured from 22nd to 42nd week of normal pregnancy and compared to published data.