Abstract
The photoaffinity label 2-nitro-5-azidobenzoyl-Gly-Trp-Met-Asp-Phe-NH2 (NAB-CCK-5) mimics cholecystokinin and the more potent secretagogue cerulein in stimulating discharge of exportable proteins from acinar cells of the guinea pig pancreas in vitro. Photolysis of this affinity label in the presence of pancreatic lobules (small clusters of acini) causes irreversible stimulation of protein discharge which is indistinguishable in rate, magnitude, and morphological aspects from that observed with the most potent doses of cerulein. The irreversible agonist activity cannot be removed by extensive washing of the lobules, and is blocked only by metabolic inhibition. Irreversible agonist activity is not observed without photolysis, nor is it observed after photolysis of lobules in the presence of 2-nitro-5-azidobenzoic acid, 2-nitro-5-azidobenzoyl-angiotensin II, 2-nitro-5-azidobenzoyl-Gly-Gly-Met-Asp-Phe-NH2, or cerulein, or with previously photolyzed NAB-CCK-5. Protection by native peptide secretagogues against irreversible labeling by NAB-CCK-5, however, could not be demonstrated. Although the biologically active sites which we have labeled appear to be those responsible for peptide secret-agogue-mediated release of exportable proteins, the absence of competition by native secretagogues for labeling shows that the mechanism of labeling NAB-CCK-5 is not understood at this time.
ACKNOWLEDGMENTS The authors are grateful to Ms. Betty Tai and Ms. Mary Lee Schaefer for excellent technical assistance.
- Copyright © 1977 by Academic Press, Inc.
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