Cystathionine y-Synthetase of SaZmoneZZa

An enzyme catalyzing the synthesis of L-cystathionine from the succinyl ester of L-homoserine and L-cysteine was isolated from a Salmonella mutant blocked in another step of methionine synthesis, and derepressed for cystathionine y-synthetase formation. The approximate molecular weight of the enzyme was 155,000 and it was unchanged after removal of tightly bound pyridoxal phosphate. In the absence of cysteine the same enzyme catalyzed the formation of cr-ketobutyrate from 0-succinyl-L-homoserine. This y elimination reaction was relatively slow, although the Km for succinylhomoserine was much lower than in the presence of cysteine. /%Mercaptopropionate inhibited elimination, as did other mercaptans with CY-, /3-, or y-carboxyl group, without changing the Km for succinylhomoserine; none of the mercaptans tested could replace L-cysteine as substrate. Only O-acetylhomoserine was found able to replace succinylhomoserine. N-Ethyhnaleimide had been shown to trap an intermediary precursor of cr-ketobutyrate during decomposition of succinylhomoserine catalyzed by another enzyme. This trapping reaction was shown to occur to a much smaller extent when the same reaction was catalyzed by cystathionine y-synthetase.