Abstract
Abstract d-Glyceraldehyde-3-14C was synthesized and administered to intact Nicotiana rustica. Degradation of the pyridine moiety of nicotine produced by these plants showed that over half of the incorporated 14C was in position 4 of the pyridine ring, and the remainder of the radioactivity was divided almost equally among carbon atoms 2, 3, and 6. Lack of randomization of the label in positions 4, 5, and 6 indicates that a nonsymmetrical compound condenses with aspartate to form the pyridine ring. Glyceraldehyde-3-14C was incorporated into the pyridine ring with a dilution of about 217 as compared with a dilution of about 300 when 14C-labeled glycerol was used as the precursor. This finding suggests that d-glyceraldehyde may be a more immediate precursor of the pyridine ring than glycerol. Incorporation of 14C into carbon atoms 2 and 3 of the pyridine ring indicates that d-glyceraldehyde is phosphorylated and metabolized through glycolysis to form acetate-2-14C which is then converted to aspartate-2,3-14C through the reactions of the tricarboxylic acid cycle.
Highlights
The dilution, the specific activity of the precursor divided by the specific activity of the nicotine isolated, of glyceraldehyde-3-14C was 217
The radiolabel from D-glyceraldehydeW4C, if metabolized via Pathway A would produce n-glyceraldehyde 3phosphate-l-14C, whereas Pathway B would result in 1% in carbon 3
Because of the participation of L-glycerol dehydrogenase in the first step of Pathway A, there are no metabolically symmetrical 3-carbon compounds in either pathway, it is conceivable that glycerol could be oxidized to dihydroxyacetone by alcohol dehydrogenase and phosphorylated to produce glyceraldehyde-3-P labeled in carbon atoms 1 and 3
Summary
The purpose of the present work was to test these two hypotheses for the incorporation of glyceraldehyde into the pyridine ring of nicotine
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