A Novel One-step Process for Enzymatic Incorporation of Amino Acids into Proteins: Papain-catalyzed Polymerization of L-Methionine Ethyl Ester and Its Regulation by Adding a Protein Substrate

Abstract

To enhance a methionine level of soy protein the plastein reaction was replaced by a simplified one-step process which had been newly developed as described in the preceding paper. It was found that during the process L-methionine ethyl ester (H•Met•OEt) underwent papain-catalyzed polymerization when the enzymatic incubation was carried out in the absence or inadequacy of soy protein. There was a critical point with regard to the ratio of the concen-tration of H•Met•OEt•HCl ([M]) and that of the soy protein ([S]). A sufficiently low [M]/[S] prevented H• Met•OEt from forming its polymer, permitting efficient incorporation of methio-nine into soy protein. A high possibility was suggested that in such a case H•Met•OEt acted as a nucleophile amine to be incorporated in the mode of papain-catalyzed aminolysis of protein.