Monoiodo-oxytocin: Demonstration of Its Biological Activity and Specific Binding to Isolated Fat Cells

Abstract

Monoiodo-oxytocin was prepared by iodinating oxytocin lightly and then freeing the iodooxytocin from uniodinated oxytocin and free iodide by gel nitration on Sephadex G.25 and chromatography on Dowex IXIO. The iodooxytocin was shown to be essentially all monoiodospecies by spectral titration. Monoiodo-oxytocin had 10-40% of the activity of oxytocin in stimulating adenylate cyclase in homogenates of toad bladder epithelium, and 75-80% activity in stimulating glucose oxidation in isolated fat cells. 125IOxytocin was bound to isolated fat cells: unlabeled oxytocin, lysine-vasopressin, and arginine-vasopressin were about equally potent in competing with 125I-oxytocin for binding. Insulin also competed but much more weakly. Glucagon and angiotensin were without significant effect. The assay can detect as little as .01 JAM of the unlabeled nonapeptide. (Endocrinology91: 1199, 1972)