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The binding of 125I somatomedin B (SMB) to proteins in serum from normal and growth hormone deficient (GHD) children

A report(1) suggests that SMB may inhibit proteases with trypsin-like activity. These proteases could be concerned with the release of biologically active growth factors (IGF) from their specific carrier proteins. The objectives of this study were to (a) examine if decreased binding of 125I SMB occurs in serum from GHD children thus reflecting an increased pool of unbound SMB. (b) To explore the binding characteristics of SMB. Purified SMB was labelled to a mean specific activity of 209 μCi/μg. Dilution curves of serum from GHD children (N = 15, 2 1/2 - 18y) and age matched controls were studied. After incubation (4°C, 2h), free from bound label was separated with ammonium sulphate. The mean percentage of label bound to diluted sera (12.5 - 50μl in the assay) from GHD children was lower than for controls (P <0.001). The characteristics of the specific binding protein were that of a low capacity (160 pmol/l) low Ka (4.37 × 106l/m) binding protein. Globulins of high capacity with immunoelectrophoretic properties of α2 macroglobulin and α1 globulins were also demonstrated. It was concluded that the specific binding of 125I SMB is low, that total binding is weak and that in consequence unbound SMB may be elevated in some GHD patients' sera.(1) Fryklund, L. et al. In: Somatomedins and growth. Academic Press, London and New York. 1979, p 7-16.