Essential arginine residues occur in or near the catalytic site of L-amino acid oxidase.

Butanedione in borate buffer irreversibly inactivates L-amino acid oxidase. L-Phenylalanine and L-methionine, which are good substrates for the enzyme, protect against inactivation but glycine, which is a very poor substrate, and D-phenylalanine which is neither substrate nor inhibitor, do not provide significant protection. These results are consistent with the modification by butanedione of one or more arginine residues located in or near the catalytic site of L-amino acid oxidase.