Studies on the biogenesis of the ring systems of nicotine.

Dawson et al. (1) have shown that the pyridine ring of nicotinic acid is utilized to form the pyridine ring of nicotine in sterile cultures of tobacco root. Since studies with animals and some microorganisms have demonstrated that tryptophan is converted to nicotinic acid (2)) tryptophan might reasonably be expected to yield the pyridine ring of nicotine. However, experiments have shown conclusively that in tobacco and some other higher plants there was essentially no conversion of C14or tritium-labeled tryptophan to compounds containing the pyridine ring (3, 4, 5). It was, therefore, necessary to examine other compounds as possible intermediates in the biosynthesis of the ring systems of nicotine. The methyl carbon of acetate, but not the carboxyl carbon, recently was shown to yield nicotine labeled significantly in the pyridine ring (6). This finding led to the postulate that in some higher plants the pyridine ring may be synthesized from simple metabolic intermediates related to acetate. The present study is an investigation of the incorporation of several W-labeled compounds, which might be interconvertible with acetate in metabolism, into nicotine in intact tobacco plants. In addition, a partial degradation of nicotine which allows for isolation of carbon from the 3 position of the pyridine ring is described.