The inhibition of D-amino acid oxidase contained in permeabilized cells of the yeast Trigonopsis variabilis by alpha-keto acids (pyruvic acid, phenylpyruvic acid and 4-methylthio-2-oxobutanoic acid), products of the transformation of the corresponding D-amino acids, was studied. In all cases, inhibition was of the mixed type and significant differences with respect to the inhibition shown by the enzyme from other sources such as pig kidney or the yeast Rhodotorula gracilis were observed. A study was also made of the thermal deactivation of the enzyme contained in permeabilized cells of T variabilis in the temperature range 30-50degreesC in sodium phosphate and Tris hydroxylmethyl aminomethane + CaCl2 buffers. A deactivation mechanism with two steps in series is proposed to account for the variation in activity with time. The results suggest that the enzyme shows greater stability in phosphate buffer, with half-lives between 7.6 days at 30degreesC and 8.6 h at 50degreesC. (C) 2004 Society of Chemical Industry.