Phytochemical Studies on the Tobacco Alkaloids

Duplicate feeding experiments of dl-ornithine-2-¹⁴C to the excised tobacco root culture were made, and the radioactive nornicotine was isolated. Approximately two thirds of the radioactivity was located in the 2-position of the pyrrolidine of the nornicotine in these experiments. This fact indicates that there are two modes in nornicotine biosynthesis: exclusive incorporation to the C-2 and equal incorporation to C-2 and C-5 from C-2 of ornithine.

On the basis of this finding, biosynthetic route was discussed.

dl-Ornithine-2-¹⁴C, dl-methionine-¹⁴CH₃ and partially racemized l-nornicotine-2,5-¹⁴C were administered to aseptically grown excised roots (N. rustica var. Brasilia). Incorporation of their radioactivity to nicotine was compared. The extent of their radioactive incorporation to nicotine was high in the order of ornithine, methionine and nornicotine; incorporation of radioactivity of nornicotine to nicotine was extraordinarily low. ¹⁵N-Labeled nornicotine was also fed to the same materials and ¹⁵N distribution was examined. Most of ¹⁵N still remained in the nornicotine reisolated. Marked amounts of ¹⁵N were located in the ethanol-insoluble fraction, the amino acid fraction and the substances having chromatographic R_F value close to that of nicotine. Only small amount of ¹⁵N was incorporated to the isolated nicotine.

Nornicotine is generally accepted to be a direct precursor of nicotine in tobacco plants. From these findings, however, it can be said that the biosynthesis of nicotine can occur through other routes without going through nornicotine.