D-Amino acid oxidase from Trigonopsis variabilis was purified to homogeneity by a combination of freezekhawing, isoelectric precipitation and chromatography on Mono Q. This purification procedure required very little working effort. The homogeneous enzyme exhibited a ratio AZXJA45U of about 6.5 and was obtained in high yield (63%) and a good stability. Using D-methionine as a substrate, a specific activity of 120 U/mg was determined colorimetrically at 26 "C, corresponding to 185 U/mg polarographically at 37°C. Polyclonal antibodies were raised against the homogeneous protein and Western immunoblot analysis showed that the 39-kDa subunit can undergo defined cleavages at the carboxy terminus of amino acid positions 104, 106 and 108, leading to 27-kDa and 12-kDa fragments as revealed by SDSPAGE, which are still enzymically active in their native form. The enzyme was inactivated by all sulfhydryl-modifying reagents tested. Inactivation by 5,5'-dithiobis(-2-nitrobenzoate) was correlated with a modification of up to 2 mollmol protein of the six cysteine residues present in the monomer. Identification of the most reactive cysteine was achieved by inactivation of the enzyme with the fluorescent, sulfhydryl-modifying reagent monobromobimane. In the presence of a substrate amino acid, under anaerobic conditions, the protein could be protected from modification and, thus, inactivation by this reagent. Peptide mapping by reversephase chromatography of endoproteinase Glu-C-digested monobromobimane-labeled enzyme revealed one major fluorescence peak which was not obtained when the protein was modified in the presence of a substrate amino acid under anaerobic conditions. Isolation and sequencing of the labeled peptide led to the identification of Cys298 as the reactive cysteine residue. D-Amino acid oxidase catalyzes the oxidation of D-amino acids to their corresponding 2-imino acids which are subsequently hydrolysed non-enzymically to 2-0x0 acids and ammonia. Reoxidation of the enzyme by molecular oxygen is accompanied by the release of H,O,. The enzyme contains one FADlsubunit and belongs to the class of dehydrogenases/ oxidases [l]. This protein is of special importance in research and biotechnology 12-51 and was, therefore, subject to a diversity of investigations (for review see [2]). For a long time, the enzyme from pig kidney has been the only source of homogeneous D-amino acid oxidase 161. Recently, the enzyme from the yeast Rhodotorulu gracilis was purified to homogeneity and its properties were determined [7 -101. D-Amino acid oxidase activity was also detected in the yeast Trigonopsis vuriubilis, and the enzyme was purified to near homogeneity, but in contrast to R. gracilis the enzyme preparation showed a very low stability [ll]. Although the amino acid sequence of the enzyme is published [12] Chemical modification of D-amino acid oxidase from pig kidney by amino-acid-specific reagents led to the identification of histidine [15, 161, arginine [17], tyrosine [16, 181, lysine [18], methionine [19] and cysteine [20...