BIOLOGICALLY ACTIVE LUTEINIZING HORMONE (LH) IN PLASMA

Abstract

A comparison was made between an in vitro bioassay method and a radioimmunoassay (RIA) method for the assay of LH activity of male plasma. The in vitro bioassay was based on the LH-stimulated production of testosterone by interstitial cells obtained from mouse testes. The RIA procedure consisted of a non-equilibrium procedure employing an antiserum with high specificity and a well characterized iodinated h[human]LH tracer. A highly purified h[human]LH preparation (hLH 1st IRP [International Reference Preparation] for immunoassay) was used as standard with both procedures. Parallel line assays were performed with both methods on 36-38 plasma samples obtained over 24 h from each of 7 normal male subjects. Significantly higher concentrations of bioactivity than immunoactivity were obtained with a biological : immunological (B/I) ratio for all subjects of 1.56 (1.34 : 1.78) (geometric mean and 95% confidence limits in brackets). A regression analysis of the 2 activities for each subject indicated a close correlation (r [correlation coefficient] = 0.90) and the 95% confidence limits of the regression line intercept obtained from all subjects included the origin. Significant differences between subjects were observed with regard to the B/I ratios and regression coefficients. A comparison of the B/I ratio for male plasma (1.56 (1.34:1.78)) with that of a highly purified hLH preparation (1.39 (1.34:1.44)) suggests that the elevated ratios found in male plasma are in part attributable to the presence in the standard preparation of relatively high levels of immunologically active material which is biologically inactive. If so, a reference preparation with higher purity would result in more similar values of biological and immunological activity. With the use of suitable assay methods a close relationship can be shown between the biological and immunological activities in male plasma. The significance of the differences in the B/I ratios and the regression coefficients observed requires continued investigation.