抗ガン性酵素メチオニン　γ-リアーゼに関する研究

Methionine γ-lyase, a pyridoxal-p dependent enzyme which was purified from Pseudomonas putida catalyzes α, γ-elimination and γ-replacement reactions of L-methionine and its derivatives, and also α, β-elimination and β-replacement reactions of S-substitude cysteines. The unique catalytic mechanism of methionine γ-lyase was studied using 1-vinylglycine and the mechanisms of inactivations were studied using suicide substrates, L-propargylglycine, S-(N-methylthiocarbamoyl)-L-cysteine and L-2-amino-4-pantanoate. The enzyme also catalyzes the rapid exchange of the α-and β-hydrogens of methionine and other amino acids with deuterrium from solvents. From these studies, mechanisms for α-and β-hydrogens of the substrate amino acids are initially removed, and then the γ-substitute is eliminated to yield a vinylglycinepyridoxal-P intermediate, which is a common key intermediate in α, γ-elimination and γ- replacement reactions. In addition, the gene encoding this enzyme was cloned and the primary structure of the enzyme was deduced from its nucleotide sequences. The methionine γ-lyase gene was expressed in Escherichia coli. We found a part of an open reading frame (termed mdeB) in the 3'-franking region of the L-methionine γ-lyase gene, suggesting the presence of an operon involved in methionine catabolism. The deduced amino acid sequence of MdeB showed a high homology with the N-terminal region of E1 component of pyruvte dehydrogenase complex from E. coli. We purified and characterized the novel α-ketobutyrate decarboxylase(MdeB) from E. coli transformant. Some of its properties were described.