Interaction between 1,4-thiazine derivatives and d-amino-acid oxidase

Abstract

Aminoethylcysteine-ketimine (2H-1,4-thiazine-5,6-dihydro-3-carboxylic acid) strongly inhibits d-amino-acid oxidase (d-amino-acid: oxygen oxidoreductase (deaminating), EC 1.4.3.3). The inhibition is purely competitive (Ki = 3.3·10-7 M). Aminoethylcysteine-ketimine modifies the visible spectrum of the enzyme: the absorption maxima of bound FAD shift from 375-455 nm to 385-445 nm with a definite shoulder at 465 nm; the appearance of a large absorption band centered at 750 nm may be due to a charge-transfer complex formation. The dissociation constant for the aminoethylcysteine-ketimine-enzyme complex, calculated by a photometric procedure (4·10-7 M), is in good agreement with kinetic data. The dicarboxylic analogue of this inhibitor (lanthionine-ketimine) is ineffective in d-amino-acid oxidase inhibition and does not produce any spectral modification of the enzyme. These results confirm structural requirements for d-amino-acid oxidase inhibitor reported by other researches. Ketimine reduced forms (thiomorpholine-2-carboxylic acid and thiomorpholine-2,6-dicarboxylic acid) are chemically synthesized and checked as d-amino-acid oxidase substrates: only thiomorpholine-2-carboxylic acid is oxidized to aminoethylcysteine-ketimine (Ki = 2·10-4 M). © 1983.