Immunoreactive Somatomedin A in Human Serum*

Abstract

A RIA has been developed for somatomedin A (SM-A) utilizing Sepharose-bound antibodies. This assay, measuring SM-A, the insulin-like growth factors 1 and 2, and somatomedin C, allows determination in serum samples. In comparison with a serum standard, the mean serum levels in patients with acromegaly or GH deficiency and healthy subjects were 8.7 %/- 0.7 (n = 25), 0.24 +/- 0.02 (n = 25), and 1.15 +/- 0.11 U/ml, respectively. The correlation coefficient between immunoreactive SM-A and SM-A by radioreceptor assay was highly significant (r = 0.93), although the potency ratio of SM-A between the two groups of patients was higher in the RIA than in the radioreceptor assay. Gel chromatography revealed that SM-A in acromegalic serum is bound to a carrier protein which is absent in patients with GH deficiency. After gel chromatography at low pH, 90% of applied immunoreactive SM-A was recovered in the low molecular weight fraction and consisted mainly of neutral polypeptides.