Radioreceptor Assay of Insulin-Like Peptides in Human Plasma: Growth Hormone Dependence and Correlation with Sulfation Activity by Two Bioassays*

Abstract

A radioreceptor assay (RRA) for a slightly acidic insulin-like peptide (ILAs) with the properties of a somatomedin has been developed. The current studies document the apparent GH dependency and sulfation activity of plasma ILAs RRA reactivity. Acid gel filtration of plasma on Sephadex G-150 has been utilized to separate small molecular ILAs from binding proteins before assay by RRA. Sulfation activity (SA) has been assessed in whole plasma from subjects with variable GH status by porcine cartilage bioassay (somatomedin) and by analysis of column eluate pools in a cultured chondrocyte bioassay after acid chromatography of plasma. Acid gel filtration of plasma revealed two major peaks of ILAs RRA reactivity. The larger molecular weight peak (K_av, 0.2–0.4) was heat labile, nonparallel to the ILAs RRA standard, and lacked SA. This peak, thought to be due to binding proteins, was increased in acromegaly and decreased in hypopituitarism. A small molecular weight peak of ILAs reactivity (K_av, 0.6–0.9) demonstrated potent SA that varied with endogenous GH status. An intermediate-sized ILAs reactivity (K_av, 0.40–0.60) was observed in subjects with GH and PRL adenomas but not in normal subjects. Only the intermediate peak of acromegaly has been tested, and this demonstrated potent SA. The significance of this observation is uncertain. Somatomedin activity of whole plasma correlated well with the amount of small molecular weight ILAs measured in low GH states but did not correlate well with the amount measured in patients where GH or PRL levels were elevated due to pituitary tumors. Administration of GH for 8 days produced a dramatic increase in small molecular weight ILAs in four older hypopituitary subjects, whereas a young infant showed an increase after only 28 days. One subject showed growth inhibition due to cortisone despite increased ILAs reactivity induced by GH.