Circulating forms of somatostatinlike immunoreactivity in human plasma

Abstract

Circulating forms of somatostatinlike immunoreactivity (SLI) in humans were characterized using several chromatographic techniques. After gelfiltration chromatography on Bio-Gel P-6 columns greater than 90% of circulating SLI was of high molecular weight (MW) and eluted in the void volume. When plasma samples were passed through protein A-Sepharose columns, more than 85% of the high MW SLI was removed, indicating that this form of plasma SLI is mainly due to cross-reacting immunoglobulins. Extraction of 10-ml plasma samples from normal subjects on octadecyl silyl silica cartridges eliminated the high MW material. In addition, this extraction technique concentrated the two lower MW forms of SLI, which coelute on gel filtration chromatography with somatostatin-28 (S-28) and the tetradecapeptide form of somatostatin (S-14), respectively. Extracted plasma SLI was further analyzed by high-pressure liquid chromatography (HPLC). The results confirmed the identity of S-28 and demonstrated that S-14 is converted, in part, to Des-Alasomatostatin (S-13) following secretion into the circulation. At least four forms of SLI are thus present in human plasma: cross-reacting immunoglobulins, S-28, S-14, and S-13. Concentrations of SLI forms in the plasma of normal controls and patients with renal failure or cirrhosis were measured to assess the role of circulating somatostatin in health and disease. High MW SLI was elevated above normal in the plasma of patients with cirrhosis, but was not significantly elevated in patients with chronic renal failure. On the other hand, concentrations of plasma S-28 and S-13/14 (total concentrations of S-13 plus S-14) were elevated in patients with either chronic renal failure or cirrhosis.