416 SOMATOMEDIN BINDING PROTEIN IN GROWTH HORMONE DEFICIENCY

Abstract

The somatomedin (SM) peptides are carried in plasma complexed to SM binding proteins (SMBP). The SM complexes have two major forms in plasma. The 150K dalton complex is made up of three subunits: an acid stable SMBP, SM peptide, and an acid labile component. The 40K complex is made up of a SMBP and SM peptide In addition, there is unsaturated SMBP found mainly in the 35-40K region. We have compared levels of unsaturated SMBP in 7 normal adults to 21 children with growth hormone deficiency before and during treatment with hGH 0.1U/Kg/day 4 days. The SM-C/IGF-I content of each plasma was measured by RIA. There was a significant (p< 0.01) difference in unsaturated SMBP between normal controls (17.8±0.8 %Bound/20ul) and untreated hypopituitary patients (27.8±2.2 %Bound/20ul). Thus a lower SM-C/IGF-I was associated with a higher unsaturated SMBP. Furthermore, there was a significant negative correlation between SM-C/IGF-I content and unsaturated SMBP in the untreated hypopituitary patients (r=0.73, p< 0.001). Treatment with hGH normalized the mean unsaturated SMBP in the hypopituitary patients within two days. Full displacement curves and Scatchard analysis showed that the increased unsaturated SMBP in hypopituitary plasma was entirely due to an increased affinity (8.4±0.9 × 10−10M) when compared to normal (2.3±0.2 × 10−9M). We conclude that a higher affinity form of unsaturated SMBP is uniquely present in hypopituitarism which disappears with GH treatment. This change is in contrast to the low levels of acid stable SMBP which increase after GH.