Identification of human semen insulin-like growth factor-I/somatomedin-C immunoreactivity and binding protein

Abstract

Human seminal plasma (SP) samples were tested for insulin-like growth factor-I/somatomedin C (IGF-I/SM-C) immunoreactivity. Gel chromatography of SP at neutral pH indicated that all immunoreactive IGF-I/SM-C was present at a higher MW than the free peptide from plasma, while in 1 M acetic acid, a major peak of immunoreactivity was seen at an apparently lower MW than the free peptide, together with a peak with MW about 40,000. This latter peak contained a binding protein which, on Scatchard analysis, was shown to have a single class of binding site, Ka = 1.2 .times. 1010 l/mol. The low MW peak material was displaced from monoclonal and polyclonal IGF-I/SM-C antibodies parallel to standard preparations. Its elution on gel chromatography later than plasma IGF-I/SM-C was shown by re-chromatography to be due to physical retardation on the column, rather than a lower MW. As acid-ethanol extraction of SP samples failed to remove binding activity, samples from patients were all subjected to acid gel chromatography before assay. The mean IGF-I/SM-C content in 5 normal men was 20.7 .+-. 3.7 (SD) ng/ml while that for 4 azoospermic subjects was 9.2 .+-. 3.7 ng/ml. A hyposomatotropic subject with normal sperm density and low serum IGF-I/SM-C had an SPIGF-I/SM-C value in the normal range. Human semen contains protein-bound immunoreactive IGF-I/AM-C which is at least in part of testicular origin, and apparently not growth hormone-dependent.