Experiments on the Biogenesis of the Pyridine Ring in Higher Plants.

That tryptophaln may be a precursor of nicotinic acid in rats and Neurospora was shown in the former by feeding experiments with niacine-deficient animals (13) and in the latter by studies with mutants requiring 3-hydroxyanthranilic acid (1), a metabolite known to arise from tryptophan. The mammalian experiments have been substantiated with isotopic experiments (8, 7) wvhere tryptophan-3-C14 and 3hydroxyanthranilic-7-C14 acid have resulted in the excretion of N--methylnicotinamide-7-C'4. Equivalent experiments with higher plants do not 'drovi(e similar results. For example, in excised peas, wvhere a steady state of nicotinic acid exists, so that precursors of nicotinic acid accumulate as trigonelline (N-methylnicotinic acid), no accumulation of the latter occurred when the plants were fed ornithine, tryptophan, citrulline or pyridoxine (22). 'More directly, the trigonelline from garden peas fed trytophan-3-C14 was inactive (11). We have also synthesized 3-h-droxyanthranilic acid-7-C'4 and fed them to excisedl soybean leaves. The lack of conversion of this compoun(l to trigonelline is reported here. Grafting experiments by Dawson (2) have shown that nicotine [,8(a-N-methylpyrrolidyl) -py-ridine] is synthesized solely by the roots of the tobacco plant, while its isomer, anabasine [L8-(a-piperidyrl)-pyridine] is synthesized both in roots and leaves. Independent studies (3, 9) have shown that the pyrrolidine ring of nicotine may be synthesized from ornithine via a symmetrical precursor (e.g., putrescine or pyrrolidine), which, however, contributes no activity to the pyridine portion of the ring. The conversion of lysine to pipecolic acid occurs in both plants and animals(6,