Metabolism of the Methyl Group of Nicotine in Nicotiana rustica

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Abstract
The rate of methyl group metabolism in N. rustica var humilis was studied by hydroponically feeding mature plants either methionine-methyl-C14 or nicotine-methyl-C14 and isolating nicotine from the plants after 6 hours to 9 days. In the case of labeled methionine feeding, a major portion of the C14 entering the methyl group of nicotine did so within 2 days. Thereafter the total amount of C14 in the nicotine showed only a slight increase up to 9 days, the highest value attained being 5.5% of the radioactivity fed. When nicotine-methyl- C14 was administered, only 38% of the isotope remained in the nicotine isolated after 2 days. The amount of radioactivity in nicotine then leveled off and remained fairly constant up to 9 days. Those plants without roots given labeled nicotine metabolized methyl groups to the extent of 39% of the total nicotine methyl metabolism in whole plants. Plants without roots given labeled methionine showed little incorporation of the isotope into nicotine. Apparently the nicotine methyl formation that takes place in the aerial parts of the plant does not involve methionine. Plants without roots that had been fed ring-labeled nicotine had essentially no ability to metabolize the labeled compound, whereas those with roots metabolized 48% of the isotope in 1 day. It appears that transmethylation from methionine to nicotine as well as catabolism of the ring structures of nicotine occur almost exclusively in root tissue in N. rustica.