Comparative review of the recent enzymatic methods used for selective assay of l-lysine

l-Lysine is an essential amino acid important for maintaining human health. To date, many enzymatic methods for assay of l-lysine have been developed. The first method has been developed using l-lysine α-oxidase (l-LysOα). However, low specificity towards l-lysine of l-LysOα is a disadvantage inherent in this method. Recently, methods more specific to l-lysine were developed using newly discovered enzymes such as l-lysine ε-oxidase (l-LysOε), l-amino acid oxidase/monooxygenase (l-AAO/MOG) and l-lysine decarboxylase/oxidase (l-Lys-DC/OD). The present paper reviews recent enzymatic methods used for assay of l-lysine. These l-lysine selective assays rely on detecting and quantifying hydrogen peroxide, a product generated by the oxidase reaction of these enzymes. l-LysOε catalyzes the oxidative deamination of the ε-amino group of l-lysine, thus assays using this enzyme are more specific towards l-lysine than the ones using l-LysOα. The l-AAO/MOG has high substrate specificity towards l-lysine; however it exhibits l-lysine oxidase and monooxygenase activities. The sensitivity of l-AAO/MOG method was improved either by using its mutant, which has reduced monooxygenase activity, or by coupling with an aminoamide-oxidizing enzyme. The l-Lys-DC/OD exhibits both l-lysine decarboxylase and oxidase activities. The sensitivity of the l-Lys-DC/OD method was improved by using putrescine oxidase to oxidize the decarboxylation product of l-lysine.