Diastereospecific, enzymically catalysed transmethylation from S-methyl-L-methionine toL-homocysteine, a naturally occurring process

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Abstract
A known catabolic pathway of S-methyl-L-methionine in higher plants: donation of a methyl group to L-homocysteine resulting in the production of two molecules of L-methionine, is subjected to stereochemical studies. The two, diastereoisomeric (2-2H, methyl-13C)-S-methyl-L-methionines are synthesized and utilised in transmethylation reactions with L-homocysteine as the acceptor and an enzyme preparation from jack beans as a catalyst. The resulting, variously labelled methionine species are converted into butyl esters of the N-trifluoroacetylated derivatives and, as such, subjected to g.l.c. combined with mass spectrometry in two ionisation modes. Experimentally determined parameters such as mass peak intensities, isotopic enrichment factors, diastereoisomeric purities, and protein-derived methionine, are utilised for calculating the stereoselectivity in the enzyme transfer of the diastereoisotopic methyl groups from S-methyl-L-methionine to L-homocysteine. Together, the independent results from the two series of diastereoisomers reveal an enzymic preference of the pro-(R)-methyl group to the extent of 94% or more.