The Pyridine Nucleotide Cycle and Its Role in the Biosynthesis of Ricinine by Ricinus communis

The pyridine moieties of the pyridine nucleotides, nicotinic acid mononucleotide, nicotinic acid adenine dinucleotide, and nicotinamide adenine dinucleotide, were shown to be incorporated into ricinine by Ricinus communis L. with an efficiency comparable to that of quinolinic acid, nicotinic acid, and nicotinamide, which had been established previously as being efficient precursors of this alkaloid. All of these pyridine compounds except quinolinic acid were postulated to be closely related in a cyclic form, and they could function as a source of metabolic intermediates for the biosynthesis of ricinine. It was shown that nicotinamide was readily converted to nicotinic acid and N-methylnicotinamide in vivo; nicotinic acid was converted to nicotinamide adenine dinucleotide and N-methylnicotinic acid (trigonelline) in vitro; nicotinamide adenine dinucleotide was readily converted to nicotinamide and N-methylnicotinamide in vivo.