Biochemical studies in tobacco plants. I. The relative utilization of the optical isomers of α- and β-methionine through transmethylation in Nicotiana rustica

Abstract

In order to investigate the relative utilization in the synthesis of the N-methyl group of nicotine, and the O-methyl groups of cell-wall constituents, l-α-, d-α-, l-β-, and d-β-methionine-methyl-C 14 were fed in separate experiments to the intact Nicotiana rustica plant. Nicotine was isolated separately from the roots and from the leaves with stem at various time intervals. In all cases nicotine was found radioactive, all the radioactivity being located in the N-methyl group. Under identical conditions, l-α-methionine was utilized to a much greater extent (5–10 times) than its enantiomorph. The extent of utilization of d-α- and l-β-methionine was similar, whereas that of d-β-methionine was distinctly lower. Demethylation of plant tissue residues showed that the highest incorporation of activity into the O-methyl carbon of cell-wall constituents was obtained again with l-α-methionine. In decreasing order, d-α-methionine and then both β-methionine isomers followed. In all cases the incorporation of radioactivity into O-methyl groups was higher in the leaves with stem than in the roots. The results indicate a stereospecificity in the transmethylation reactions in tobacco plant metabolism.