Insulin-like growth factor-I and -II and their binding proteins in human ejaculates

The insulin-like growth factor (IGF)/IGF-binding protein (IGFBP) system was investigated in ejaculates from thirty men aged 23-46 years. Mean ejaculate volume was 3.3 mL (range 1-6 mL). Viscosity was increased moderately in six samples and greatly in four more. The number of motile sperm (0-165 million/mL; mean 49; median 39) was highly correlated with total number (P < 0.001). Concentrations of IGF-I and -II determined by displacement radioimmunoassay were positively correlated (P = 0.001). IGF-I ranged from 2.7 to 29.7 nmol/L (mean 9.6; median 6.5) and increased with viscosity. IGF-II ranged from 5.7 to 39.0 nmol/L (mean 16.0; median 13.5). IGFBP-2 and putative IGFBP-4 were detected by autoradiography of ligand blots after polyacrylamide gel electrophoresis. These binding proteins were stable in ejaculates stored at -18 °C with protease inhibitors and remained associated with higher molecular weight complexes during molecular size chromatography with 1 mol/L acetic acid. They eluted at least partially in the expected fractions with 1 mol/L sodium chloride but further attempts at purification were unsuccessful due to progressive degradation. It is suggested that ionic bonds between IGFBP-2 and other components, such as proteoglycans, provide protection from proteolysis in a similar way as shown for IGFBP-1 and alpha2-macroglobulin.