Effect of proteolytic enzymes on masked insulin receptors in rat submaxillary gland microsomes

Trypsin and alpha-chymotrypsin effects on masked insulin receptors were studied. Phospholipase C treatment, incubation in a high ionic strength buffer or solubilization were used as alternative procedures for the unmasking of insulin receptors. These three methods expose receptor structures which are inaccessible to insulin in the current experimental conditions of binding assays without any significant change in binding affinity. Both exposed and masked receptors proved to be equally sensitive to trypsin and alpha-chymotrypsin degradation. At 25 degrees C, about 5 micrograms trypsin/ml for 50 min or 80 micrograms alpha-chymotrypsin/ml for 200 min were necessary in each case to cause a 50% inhibition of the binding of 125I-iodo insulin to microsomes. The results suggest that masked receptors are only nonfunctional to bind insulin but they are not located in compartments inaccessible to molecules present in the medium.