Incorporation of Aspartate and Malate into the Pyridine Ring of Nicotine

A unique nicotinic acid degradation was devised which permitted the isolation of each individual carbon atom of the pyridine ring. Aspartate-3-14C and malate-3-14C were administered to Nicotiana rustica tobacco plants for 4 hours and found to be incorporated almost exclusively into carbons 2 and 3 of the pyridine ring of nicotine. When aspartate-3-14C was fed, 57% of the tracer resided at carbon 3 and 38% at carbon 2. Malate-3-14C labeled carbon 3 with 61% of the isotope and carbon 2 with 38%. The dilution of incorporation of aspartate-3-14C was 600 and of malate-3-14C, 1260. The incorporation of aspartate and malate with low dilution and the lack of randomization of 14C between carbons 2 and 3 of the pyridine ring indicate either that aspartate or malate is the immediate precursor of the pyridine ring of nicotine, or that they are converted to the immediate precursor without passing through a symmetrical intermediate.