Studies on polycysteine peptides and proteins. I. Isomeric cystinylcystine peptides

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Abstract

l-Cystinyl-l-cystine, l-cystinyl-d-cystine, and d-cystinyl-l-cystine were synthesized by the mixed anhydride procedure involving the reaction of carbobenzoxy-S-benzylcysteine and S-benzylcysteine ester in the presence of isobutylchlorocarbonate and triethylamine, followed by the successive removal of ester, carbobenzoxy, and S-benzyl groups, and by the oxidation of the respective cysteinylcysteines at pH 6.5 and pH 8.5 to the corresponding cystinylcystines. The peptides were all crystalline.

The l-cystinyl-d-cystine and d-cystinyl-l-cystine, except for their optical enantiomorphism, appeared to be identical and independent of the pH at which they were formed by oxidation. They yielded optically inactive cystine on HCl hydrolysis, the greater part of which appeared to be dl-cystine. dl-Cystine could be distinguished from meso-cystine by the considerably greater susceptibility of the former to the action of snake venom l-amino acid oxidase, although both forms were nearly equal in their susceptibility to cystine desulfurase.

The l-cystinyl-l-cystine formed by oxidation at pH 6.5 appeared by several criteria to be different from that formed by oxidation at pH 8.5, although their elemental analyses were practically identical, and it is possible that the conditions of oxidation led to configurational isomers. Both forms on HCl hydrolysis yielded pure l-cystine. The dihydrochloride salts and the dibenzoyl derivatives prepared from the two forms of l-cystinyl-l-cystine showed in some characteristics their origin from different parent compounds, although in these cases too, the elemental analyses were practically identical.

All of the cystinylcystine peptides were completely and quantitatively hydrolyzed by renal aminopeptidase. The hydrolytic rates for the two l-cystinyl-l-cystine forms were practically the same, and considerably higher than the nearly equal values for l-cystinyl-d-cystine and d-cystinyl-l-cystine. Renal acylase I acted very slowly on the former, and not at all on the latter peptides.

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1

Fulbright and Smith-Mundt Scholar; on leave from Kyushu University, Japan.

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