Interaction of porcine pancreatic colipase with a nonionic detergent, Triton X-100: spectrophotometric studies

Strong perturbation of the ultraviolet spectrum of the tyrosines of porcine pancreatic colipase A is observed in the presence of Triton X-100 at concentration above the critical micellar concentration. Spectrophotometric titration of the phenolic groups of the protein shows that the apparent pKa value for two tyrosines is about 10.3, while the third tyrosine has a higher pKa value above 11.6. This residue is still protonated at pH 13 in the presence of Triton X-100. All perturbations induced by the nonionic detergent can be interpreted as resulting from interactions between colipase and Triton X-100 molecules at a hydrophobic site of the protein that includes the tyrosine residues. Results obtained in studies with Triton X-100 are similar to those already reported by Sari et al. (Eur. J. Biochem. 58:561 (1975) on the interaction of colipase with taurodeoxycholate. It is likely that the binding of both types of detergent occurs at the same specific site on the protein molecule. Data presented in this communication give further support to the hypothesis that a hydrophobic domain (residues 49-57), including all three tyrosines of the colipase molecule, participate to the well characterized interaction of the lipase cofactor with triglycerides at lipid-water interfaces.