Photoaffinity Labeling of Peptide Hormone Binding Sites

The known binding of a derivative of the COOH-terminal tetrapeptide of gastrin with high biological activity to bovine serum albumin suggests a convenient model system for testing the affinity labeling of hormone binding sites. Photolysis of 4-azidobenzoylpentagastrin, 2-nitro-5-azidobenzoylpentagastrin, 4-acetylbenzoylpentagastrin, and 4-benzoylbenzoylpentagastrin in the presence of bovine serum albumin attaches each peptide derivative covalently to the protein to the extent of 50 mole % using a 3-fold excess of peptide and moderate photolysis times. Under similar conditions using lysozyme for the protein, only 8 mole % peptide is bound. Oleic acid was found to compete for the binding sites on serum albumin. When serum albumin labeled with 4-azidobenzoylpentagastrin was split with cyanogen bromide most of the label was found on two of the peptides located in the middle of the albumin chain. A degree of specificity is thus indicated. These results suggest that the peptide derivatives should be capable of labeling their binding sites on tissue which is stimulated by the hormones gastrin and pancreozymin. In addition, the model photoreactions of acetophenone and benzophenone with glycine were investigated and the photoaddition products isolated. The results of these model reactions and the successful labeling of albumin with ketone derivatives of pentagastrin suggest that ketones may be useful photoaffinity probes.