The acid-stable subunit of the GH-dependent large mol wt somatomedin-binding protein (SmBP) was isolated from human plasma Cohn fraction IV by a three-step procedure, and a specific RIA was developed which allowed measurement in unextracted serum. Although in normal human serum most of immunoreactive material was present as the large mol wt complex (150K), considerable amounts of smaller components were found by high performance liquid exclusion chromatography in the 60K, 42K, and 32K range. Normal serum levels were low at birth, rose sharply during the first weeks of life, and showed a moderate peak at puberty. To assess the diagnostic efficacy of SmBP for GH deficiency (GHD), patients previously diagnosed as GH-deficient by conventional criteria (n = 132) were compared to short statured children without GHD (n = 130). Taking the fifth centile as a limit of normality the majority of patients with GHD had subnormal levels, yielding high sensitivity of the test (0.97). In contrast, most of the non-GH-deficient children had SmBP levels within the normal range, resulting in high specificity (0.95) and, consequently, high accuracy (0.96). These results suggest that the large mol wt SmBP is an excellent screening parameter and is highly informative for GHD.

Insulin-like growth factors (IGFs) and their binding proteins (IGFBPs) were studied in children with end-stage renal failure (ESRF, n = 31) and chronic renal failure (n = 11) with residual glomerular filtration. Somatomedin bioactivity in patient sera was found to be decreased while IGF-I and IGF-II levels by radio-immunoassay (RIA) were normal. In contrast, IGFBP-1 and IGFBP-3 levels (measured by RIA) were markedly increased in uraemia. Excess IGFBP was shown to be able to bind IGF by determination of the free IGF binding capacity. Using high-performance liquid chromatography a shift of the IGFBP-3 profile to low molecular weight components could be demonstrated in ESRF. Affinity cross-linking experiments showed that these low molecular weight IGFBP-3 immunoreactive forms are biologically active. In normal urine only IGFBP-3 forms smaller than 60 kDa were detected with a major peak at 12-20 kDa. Removal of excessive IGFBP from patient sera by affinity chromatography on an IGF-II Sepharose column resulted in a significant increase in somatomedin bioactivity. Model calculations on the interaction of IGF and IGFBP using empirical data suggested a reduction of IGF secretion in uraemia by an order of magnitude. It is concluded: (1) that renal failure causes an accumulation of low molecular weight IGFBP, (2) that the resulting excess of IGFBP acts as a somatomedin inhibitor, and (3) that in uraemia there is a relative growth hormone resistance with respect to IGF production.

The insulin-like growth factors (IGF)-I and -II are bound to specific carrier proteins in the circulation. For investigation of their physiological role, the acid-stable subunit of the major binding protein (SmBP) was isolated from human plasma Cohn fraction IV. Its effect on the mitogenic activity of IGF-I was studied with baby hamster kidney fibroblasts (BHK-21) and human skin fibroblasts. While free IGF-I had no effect on thymidine incorporation into DNA with BHK-21 cells and only a moderate effect with human fibroblasts under standard conditions, DNA synthesis was significantly enhanced with both cell lines if IGF-I was complexed with SmBP before the experiment. The enhancement was optimal at an approximately equimolar ratio of both peptides. In contrast to experiments in which large concentrations of IGF-I were added at the beginning, repeated addition of small quantities of free IGF-I at hourly intervals clearly stimulated DNA synthesis in BHK-21 cells. Binding studies with radiolabeled SmBP revealed no evidence for direct interaction with either cell line. It is concluded that SmBP acts as a reservoir, releasing continuously low amounts of IGF-I and thereby creating a steady state situation of receptor occupancy, which appears to be a better mitogenic stimulus than temporary large concentrations of IGF-I.

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