CYSTATHIONINURIA AND B₆DEPENDENCY

Purification and characterization of human hepatic cysteine-conjugate β-lyase

“…The reaction (Scheme 2) is quite similar to that catalysed by human cystathionase, which cleaves a C-S linkage ofL-homocysteine, L-methionine, L-cystathionine, L-homoserine and L-cysteine by aflor ay-elimination reactions in the presence of pyridoxal 5'-phosphate, whereby the corresponding thiol product, a-oxo acid and ammonia are stoichiometrically produced. Several lines of evidence, however, clearly indicate that the present ,J-lyase is distinct from human hepatic cystathionase: (1) the activity of the /8-lyase was completely abolished by means of thermal treatment (60°C for 2 min), whereas a salient feature of the cystathionase is its higher thermostability (Frimpter et al, 1969); (2) the substrate specificity of the /)-lyase was different from that of human cystathionase; thus cystathionase acts on L-homoserine and L-cystathionine, neither of which is attacked by the present enzyme; (3) the ,-lyase and the cystathionase displayed different distribution profiles in the (NH4)2SO4 fractionation of the 105000 g supernatant fluids, i.e. most of the ,-lyase activity was found in the pellet precipitated at 40-60% saturation, whereas cystathionase was not completely precipitated even at 73 % saturation (Frimpter et al, 1969);(4) thiol-blocking reagents such as iodoacetic acid had virtually negligible inhibitory effect on the /J-lyase, whereas mammalian cystathionase contains an essential thiol group on the catalytic site (Brown & DeFoor, 1974).…”