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DON S. SCHALCH, UDO E. HEINRICH, JANET G. KOCH, CAROLYN J. JOHNSON, ROBERT J. SCHLUETER, Nonsuppressible Insulin-Like Activity (NSILA). I. Development of New Sensitive Competitive Protein-Binding Assay for Determination of Serum Levels, The Journal of Clinical Endocrinology & Metabolism, Volume 46, Issue 4, 1 April 1978, Pages 664–671, https://doi.org/10.1210/jcem-46-4-664
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Abstract
Nonsuppressible insulin-like activity (NSILA), one of the GH-dependent somatomedins, is a polypeptide (∼5800 mol wt) which is largely bound to a carrier protein of approximately 70,000 Daltons. By employing 125I-labeled highly purified NSILA (∼200 mU/mg) and partially purified NSILA carrier protein derived from pooled human plasma, a competitive protein- binding assay for NSILA has been developed. By utilizing purified NSILA (3.8 μU/mg) as standard, this assay procedure is capable of measuring NSILA levels in extracts of human and rat sera under conditions of both normal and diminished GH concentrations.
In this assay procedure, the endogenous NSILA carrier protein complex is first dissociated by Sephadex G-50 chromatography in 1 N HOAc (pH 2.3). This assay is quite specific relative to a number of hormones (insulin, proinsulin, human GH, ACTH, glucagon, thyroxine, and PRL) but may detect other somatomedins (A, C, and multiplication-stimulating activity) which, like NSILA, probably crossreact with NSILA carrier protein. The recovery of known amounts of NSILA added to serum is 74.6–94.4%, and on replicate determinations the intraassay variation is 10% whereas the interassay variation is 19%. Evidence for the validity of this assay procedure is provided by its demonstration of the GH dependency of serum NSILA levels in rats. The mean (±SEM) NSILA level in normal adult rats (400 ±12 μU/ml) is significantly decreased by hypophysectomy (33 ± 2 μU/ml), but is restored to normal (446 ± 10 μU/ml) when bovine GH is administered in vivo with replacement doses of triiodothyronine and cortisol.
