Conversion of Progesterone to Testosterone in the Rat Leydig Cell1

The bioassay for LH-like hormones, based on the amount of testosterone produced in vitroby collagenase-dispersed rat Leydig cells in response to the hormone to be tested, was used to see whether a chorionic gonadotrophin (CG)-like hormone could be detected in the serum of pregnant rats. The curve of the serum “CG” concentration (i.e. in vitrotestosterone produced) during the course of pregnancy was different from that of serum LH, determined by RIA, but was very similar to that of serum progesterone. This similarity proved to be due entirely to the conversion of progesterone to testosterone by the Leydig cells, since known amounts of progesterone (10-200 ng) dissolved in hypophysectomized rats'serum were converted quantitatively to testosterone when added to the cells under the same conditions used to test the pregnancy serum. The bioassay may thus be used as a specific measure of LH-like hormones only under conditions in which it is certain that steroids which can be converted to testosterone are either removed, or are present in amounts too small to have a significant effect on the values obtained.