| Abstract: | NSILA-s (nonsuppressible insulin-like activity, soluble in acid ethanol) is a serum peptide that has insulin-like and growth-promoting activities. We have demonstrated previously that liver plasma membranes possess separate receptors for NSILA-s and insulin and have characterized the insulin receptor in detail. In the present study we have characterized the properties and specificity of the NSILA-s receptor and compared them to those of the insulin receptor in the same tissue. Both 125I-NSILA-s and 125I-insulin bind rapidly and reversibly to their receptors in liver membranes; maximal NSILA-s binding occurs at 20 degrees while maximal insulin binding is seen at 1-4 degrees. The pH optimum for NSILA-s binding is broad (6.0 to 8.0), in contrast to the very sharp pH optimum (7.5 to 8.0) for insulin binding. Both receptors exhibit a high degree of specificity. With the insulin receptor, NSILA-s and insulin analogues compete for binding in proportion to their insulin-like potency: insulin greater than proinsulin greater than NSILA-s. With the NSILA-s receptor, NSILA-s is most potent and the order is reversed: NSILA-s greater than proinsulin greater than insulin. Furthermore, six preparations of NSILA-s which varied 70-fold in biological activity competed for 125I-NSILA-s binding in order of their potencies. NSILA-s which had been inactivated biologically by reduction and aminoethylation and growth hormone were less than 1/100,000 as potent as the most purified NSILA-s preparation. Purified preparations of fibroblast growth factor, epidermal growth factor, nerve growth factor, and somatomedins B and C were less than 1% as effective as NSILA-s in competing for the 125I-NSILA-s suggesting that these factors act through other receptors. In contrast, somatomedin A was 10% as active as NSILA-s and multiplication-stimulating activity was fully as active as NSILA-s in competing for the NSILA-s receptor. Analysis of the data suggests that there are approximately 50 times more insulin receptors than NSILA-s receptors per liver cell, while the apparent affinity of NSILA-s receptors is somewhat higher than that of the insulin receptor. |
