Heat-labile prolactin immunoreactivity in normal human tissues: artifacts of peptide hormone radioimmunoassay in human tissues.

We recently demonstrated that a substantial proportion of the previously reported widespread distribution of human chorionic gonadotropin-like material in nonplacental normal human tissues is due to non-specific activity of tissue proteases (Adejuwon & Segal, 1984). The study was extended to another polypetide hormone, measured by the same general method of radioimmunoassay. Normal tissue samples of colon (n = 1) and testes (n = 5) obtained at surgery were analysed by a highly specific and sensitive radioimmunoassay (RIA) for human prolactin (hPRL). All tissues investigated appeared to contain immunoreactive hPRL-like material (6.7 +/- 5.3 ng/g weight, mean +/- s.d.) Heat at 55 degrees C for 15-20 min was found to have no effect on the immunoreactivity of purified hPRL. Exposure of normal tissues to these conditions was, however, found to completely eliminate the hPRL-like immunoreactivity. The factor present in these normal tissues giving positive radioimmunoassay results for hPRL is not identical to native hPRL. It may be due to tissue proteases interfering with the radioligand assay. True polypeptide hormone immunoreactivity must be distinguished from proteolytic activity by treatment with heat or protease inhibitor.