Extraction of somatostatin from human plasma on octadecylsilyl silica

Human plasma contains substances that interfere with the radioimmunoassay (RIA) of somatostatin-like immunoreactivity (SLI). A method has been developed for rapid, reproducible extraction of somatostatin from human plasma on octadecylsilylsilica (ODS). Hydrophobic binding of somatostatin to ODS permitted extraction of the peptide from untreated human plasma, elution of less tightly bound substances with dilute acid, and then elution of somatostatin by 80:20 acetonitrile:0.1% trifluoroacetic acid. The lyophilized extract was reconstituted to a volume of 0.5 ml prior to quantification by RIA. This 6-fold concentration resulted in an effective lower limit of detection of 7.5 pg/ml of plasma. The interassay coefficient of variation for the combined extraction and RIA was 20% (n = 10) at a mean plasma level of 15 pg/ml. Basal concentrations of somatostatin in human plasma ranged from 8 to 20 pg/ml (n = 35, mean - 13.3 +/- 0.4). Basal somatostatin levels (mean = 14.0 +/- 0.4 pg/ml) for nonobese (BMI less than 30, n = 10) were not different from values (mean = 13.3 +/- 0.7 pg/ml) observed for the obese group (BMI greater than 35, n = 17) nor from the values (n = 8, x = 15.4 +/- 1.2 pg/ml) obtained for subjects with non-insulin dependent diabetes mellitus.