Tobacco plants were fed three C'd-labeled substrates to study their role in the biosynthesis of the pyridine ring of nicotine. Glycerol-2-C14 and aspartic a~id-3-C'~ both contributed relatively large quantities of radioactive carbon to the pyridine ring, whereas propionate-3-C14 did not. Glycerol-2-C14 was incorporated into the pyridine ring to about the same extent as glycerol-1,3-C14. Partial degradation of the pyridine ring of nicotine from plants fed aspartic acid-3-C14 revealed that aspartic acid was not converted directly to the ring, since C14 was located in more than a single position in the ring. The three labeled compounds all contributed C14 to the pyrrolidine ring of nicotine. The pattern of labeling in the pyrrolidine ring suggested that propionate was converted to acetate before utilization for nicotine biosynthesis.It has recently been shown that several metabolites which are either constituents of, or closely related to, the glycolytic pathway or the tricarboxylic acid cycle contribute carbon to some pyridine ring-containing compounds synthesized by several plants. For instance, glycer01-1,3-C'~, acetate-2-C14, and propionate-2-C14 contribute radioactive carbon to the pyridine ring of nicotine in the metabolic processes in tobacco plants (Nicotiana rustica) (Griffith et al., 1960). In addition, these same precursors along with succinate are incorporated into the 2-pyridone ring of ricinine by castor plants (Anwar et al., 1961; Waller and Henderson, 1961a, b), and Ortega and Brown (1959) have demonstrated that succinate and glycerol are utilized by Escherichia coli for the synthesis of the pyridine ring of nicotinic acid.In the studies of incorporation of various metabolites into nicotine, degradations of the alkaloid from plants fed acetate-2-C14 or propionate-2-C14 have revealed that about half of the C14 of the pyridine ring was located in carbon-3. In addition, the extent of incorporation and distribution of C14 in nicotine after feeding of labeled glycerol was different than after feeding either acetate or propionate (Griffith et al., 1960). These findings suggested that glycerol contributed carbon to one part of the pyridine ring and acetate or propionate to another part.These three precursors were incorporated into nicotine with a relatively low dilution of C14, and it appeared, therefore, that they were all closely related to the precursors which were utilized directly for pyridine ring synthesis. However, the processes leading from glycerol, acetate, and propionate to the pyridine ring are not well understood, and the purpose of the present study, therefore, was to provide further information about these processes. Evidence is presented which indicates that carbon
