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 octadecyIsilyIsi1ica(ODS). Hydrophobic bindingof somatostatin to ODS permitted extraction of this 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 concentrationresultedinaneffective lower limit of detection of 7.5 pg/ml of plasma. The interassay coefficient of variationfor the combined extraction and RIA was 20%(n=10)at a meanplasma level of 15 pg/ml. Basal concentrations of somatostatinin human plasma ranged from 8 to 20 pg/ml(n=35, x=13.3 ± 0.4). Basal somatostatin levels (x=14.0 ± 0.4pg/ml) for nonobese(BMH30, n=10)were not different from values(x=13.3 ± 10.7pg/ml)observed for the obese group(BMI>35, n=17)nor from the values (n=8, x=15.4 ± 1.2pg/ml obtained for subjects with non-insulindependent diabetes mellitus.