生化学と分析生化学

概要

Efficacy of L-Cysteine as an Anti-Oxidant in Papain Catalyzed Synthesis of Oligopeptides in Organic Solvent System

Santhana Srinivasan, Shubhen Kapila, Daniel Forciniti and Paul Nam

Enzymatic peptide synthesis has drawn considerable attention for synthesis of high by-pass oligopeptide feed supplements in animal nutrition. A hardy protease, (papain) with a cysteine moiety at the active site requires the presence of an anti-oxidant in the reaction medium to ensure that the thiol group remains intact. Free cysteine has been the antioxidant of choice for papain-catalyzed synthesis of oligopeptides in aqueous systems. However, due to limited solubility of cysteine in organic solvents, it is generally not a suitable antioxidant for the synthesis of oligopeptides in biphasic solvent systems; instead, mercaptoethanol is often used. Lysine and Methionine is couple of well-known limiting amino acids that find application in cattle feed and poultry. Synthesis of co-oligopeptides of Lysine and Methionine has generally been attempted in bi-phasic solvent systems with Mercaptoethanol as an anti-oxidant. The inherent toxicity of mercaptoethanol when present even in trace amounts make its use undesirable during synthesis of oligopeptides to be used as feed supplements. Use of non-toxic antioxidants such as Cysteine would make the end product more amenable as a nutritional supplement for animal feed. Therefore efficacy of L-cysteine as an anti-oxidant was investigated during papain catalyzed oligomerization of Lys, Arg, Glu and Asp in two organic systems: a three phase micro-aqueous media consisting of n-octane, DFP and water, as well as homogeneous ACN/water mixtures. Reactions were also carried out under an argon atmosphere in the presence and absence of anti-oxidants. The results of the experiments showed that L-cysteine facilitated oligomer synthesis in both the three phase system and the ACN/water mixture. The overall oligomer yields were found to be better than 75% in the presence of L-Cysteine. Oligopeptide yields obtained through reactions carried out under the argon atmosphere were less than 20%.