D-Serine dehydratase from chicken kidney: a vertebral homologue of the cryptic enzyme from Burkholderia cepacia.

D-serine dehydratase (DSD) catalyses the conversion of d-serine to pyruvate and ammonia. d-Serine is a physiological modulator of glutamate neurotransmission in vertebrate brains. In mammals d-serine is degraded by d-amino-acid oxidase, whereas in chicken brain it is degraded by DSD, as we have recently demonstrated [Tanaka et al. (2007) Anal. Biochem. 362, 83-88]. To clarify the roles of DSD in avian species, we purified DSD from chicken kidney. The purified enzyme was a heterodimer consisting of subunits separable by SDS-PAGE but with identical N-terminal amino acid sequences. The prominent absorption at 416 nm and the inhibition of the enzyme both by hydroxylamine and by aminooxyacetate suggested that the enzyme contains pyridoxal 5'-phosphate as a cofactor. The enzyme showed the highest specificity to d-serine: the k(cat)/K(m) values of DSD for d-serine, d-threonine and l-serine were 6.19 x 10(3), 164 and 16 M(-1)s(-1), respectively. DSD was found immunohistochemically in the proximal tubules of the chicken kidney. Judging from the amino acid sequence deduced from the cDNA, chicken DSD is a homologue of cryptic DSD from Burkholderia cepacia and low-specificity d-threonine aldolase from Arthrobacter sp. strain DK-38, all of which have a cofactor binding motif of PHXK(T/A) in their N-terminal portions.