Additional observations on cholecystokinin and the vasoactive intestinal polypeptide.

During the isolation of cholecystokinin from natural sources, as well as during its bioassay, inactivation by oxidation can cause problems. We have attempted to reactivate oxidized CCK by reduction at room temperature with N-methylmercaptoacetamide, recently stated to be the reducing agent of choice for the reduction of methionine sulfoxide to methionine [22]. We have not yet been unequivocally successful in these attempts, but the results seem promising. In the case of oxidized VIP and of oxidized tetragastrin, reduction with N-methylmercaptoacetamide does seem to result in reconversion of the peptides to their preoxidation states, as evidenced by thin layer chromatography on silica gel. We have, together with A. Holmgren and A. Ehrnberg, made observations suggesting the presence in rate liver cytosol of an enzyme which catalyzes the reductive reactivation of oxidized CCK with reduced thioredoxin as the immediate hydrogen donor. In collaboration with A. Light, Purdue University, we have found that enterokinase cleaves 39-CCK and 33-CCk with release of 8-CCK and the tetrapeptide immediately preceding it in the peptide chain. The conversion of 39-CCK to 33-CCK by the action of dipeptidyl amino-peptidase I has been confirmed.