Cystathionine -Synthetase of Salmonella

CATALYTIC PROPERTIES OF A NEW ENZYME IN BACTERIAL METHIONINE BIOSYNTHESIS

From the ¹ From the Laboratory of Biochemistry, National Heart Institute, Bethesda, Maryland 20014

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 -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 -ketobutyrate from O-succinyl-l-homoserine. This elimination reaction was relatively slow, although the K_m for succinylhomoserine was much lower than in the presence of cysteine. ß-Mercaptopropionate inhibited elimination, as did other mercaptans with -, ß-, or -carboxyl group, without changing the K_m for succinylhomoserine; none of the mercaptans tested could replace l-cysteine as substrate. Only O-acetylhomoserine was found able to replace succinylhomoserine. N-Ethylmaleimide had been shown to trap an intermediary precursor of -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 -synthetase.

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