There is a growing, though still meager, knowledge about the natural occurrence, function, or importance of n-amino acids in microorganisms, plants, soils, geochemical deposits, marine sediments, and even meteorites. Improved procedures for determining L-and n-isomers in complex mixtures are partly responsible for renewed interest in n-amino acids. Currently, the most sensitive assay methods are combinations of diastereoisomer derivative-chromatographic
(1) and of enzymic-radiochemical-chromatographic
(2) procedures. Although each has advantages, the enzymic approach appears simpler for more complex mixtures. However, the enzymic approach is limited to substrates oxidizable by the assay enzyme. This has presented problems since several important n-amino acids, e.g., n-aspartic, n-threonine, n-glutamic, and n-lysine, reportedly react poorly or not at all (3,4). Previous results with an electrophoretically pure hog kidney n-amino acid oxidase (2) showed some reactivity with these "unreactive" compounds. This report describes additional efforts to find suitable reaction conditions for complete oxidation of some important unreactive or resistant n-amino acids. Considerable interest in the procedure (2) and concurrent reports (5,6) prompted dissemenation of this information.