SUMMARY: L-Cystathionine is rapidly degraded to homocysteine, pyruvate and ammonia by cell-free enzyme preparations of an auxotrophic strain of which requires pyridoxin, glycine or serine for growth. One molecule of each of the products was formed from each molecule of cystathionine throughout the course of the reaction. The preparation did not form pyruvate from L-serine and L-alanine (which were possible intermediates); it is concluded that the cleavage is a single step reaction. After precipitation with ammonium sulphate and dialysis the enzyme required both pyridoxal phosphate and magnesium ions for full activity, but no dependence on magnesium was found with preparations from another strain. The reaction was inhibited totally by cyanide and cupricions and partially by isonicotinic acid hydrazide and sulphydryl compounds (cysteine, homocysteine, glutathione). An auxotrophic strain of which grew with methionine or homocysteine, but not with cystathionine, did not contain the enzyme.


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