Comparison of [125I]Somatomedin A and [125I]Somatomedin C Radioreceptor Assays for Somatomedin Peptide Content in Whole and Acid-Chromatographed Plasma*

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Abstract
The placental membrane radioreceptor assay was used to measure the levels of somatomedin (SM) peptides in plasma. Displacement of both [25I]- somatomedin A ([125I]SM-A) and [125I]somatomedin C ([125I]SM-C) by normal whole plasma, the peptide fraction of acid-chromatographed plasma, and a partially purified, insulin-free SM preparation were compared. The peptide fraction of plasma was isolated by acid chromatography over Sephadex G-50 in 0.25 m formic acid with a yield of >90%, as determined by bioassay and [125I]SM. In the case of [125I]SM-A, the dose-response curves for whole plasma, acid-chromatographed plasma, and the standard SM preparation were parallel (P > 0.2). In contrast, for [125I]SM-C, the dose-response curves for acid-chromatographed plasma and the purified SM preparation were parallel (P > 0.2), but both differed significantly from that of whole plasma (P < 0.001). In addition, there was less variability in the assay of acid-chromatographed plasma compared to whole plasma. The results indicate that radioreceptor assay of unextracted normal plasma using [125I]SM-A is a valid measure of SM peptide concentration, while radioreceptor assay of unextracted normal plasma using [125I]SM-C, in our hands, is not. Acid chomatography of plasma before its assay is an uncomplicated procedure which allows valid and precise measurement of SM peptide content using either [125I]SM-A or [125I]SM-C.

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