LYSINOALANINE IN FOOD AND IN ANTIMICROBIAL PROTEINS

摘要

Heat and alkali treatment of food proteins widely used in food processing results in the formation of crosslinked amino acids such as lysinoalanine, ornithinoalanine, lanthion-ine, and methyl-lanthionine and concurrent racemization of L-amino acid isomers to D-ana-logues. The mechanism of lysinoalanine formation is a two-step process: first, hydroxide ion-catalyzed elimination of cysteine and serine residues to a dehydroalanine intermediate; second, reaction of the double bond of dehydroalanine with theε-NH_2 group of lysine to form a lysinoalanine crosslink. The corresponding elimination-addition reaction of threon-ine produces methyl-dehydroalanine, which then reacts with the NH_2 and SH groups to form methyl-lysinoalanine and methyl-lanthionine, respectively. The crosslinked amino acids lanthionine and methyl-lanthionine are formed by analogous nucleophilic addition reactions of the SH group of cysteine to dehydroalanine and methyl-dehydroalanine, respectively. Processing conditions that favor these transformations include high pH, temperature and exposure time. Factors which minimize lysinoalanine formation include the presence of SH-containing amino acids such as cysteine, N-acetyl-cysteine, and glutathione, dephosphorylation of O-phosphoryl esters, and acylation of ε-NH_2 groups of lysine side chains. The presence of lysinoalanine residues along a protein chain decreases digestibility and nutritional quality in rodents but enhances nutritional quality in ruminants. Protein-bound and free lysinoalanines are reported to induce enlargement of nuclei of rat kidney cells. All of the mentioned dehydro and crosslinked amino acids also occur naturally in certain peptide and protein antibiotics. These include duramycin, cinnamycin, epidermin, sub-tilin and the widely used food preservative nisin. Mechanistic rationalizations are offered for the observed antimicrobial activities of these compounds in relation to their structures. The cited findings and new research to better define the chemistry and dietary and antimicrobial roles of lysinoalanine and related compounds should lead to better and safer foods.