Activation of the de Novo Pathway for Pyridine Nucleotide Biosynthesis Prior to Ricinine Biosynthesis in Castor Beans

Abstract

The ricinine content of etiolated seedlings of Ricinus communis increased nearly 12-fold over a 4-day period. In plants quinolinic acid is an intermediate in the de novo pathway for the synthesis of pyridine nucleotides. The only known enzyme in the de noro pathway for pyridine nucleotide biosynthesis, quinolinic acid phosphoribosyltransferase, increased 6-fold in activity over a 4-day period which preceded the onset of ricinine biosynthesis by 1 day. The activity of the remainder of the pyridine nucleotide cycle enzymes in the seedlings, as monitored by the specific activity of nicotinic acid phosphoribosyltransferase and nicotinamide deamidase, was similar to that found in the mature green plant. In the roots of Nicotiana rustica, where the pyridine alkaloid nicotine is synthesized, the level of quinolinic acid phosphoribosyltransferase was 38-fold higher than the level of nicotinic acid phosphoribosyltransferase, whereas in most other plants examined, the specific activity of quinolinic acid phosphoribosyltransferase was similar to the level of activity of enzymes in the pyridine nucleotide cycle itself. A positive correlation therefore exists between the specific activity of a de novo pathway enzyme catalyzing pyridine nucleotide biosynthesis in Ricinus communis and Nicotiana rustica and the biosynthesis of ricinine and nicotine, respectively.Numerous in vivo studies have established that the pyridine moiety of quinolinic acid, nicotinamide, and nicotinic acid is directly incorporated into the alkaloids ricinine and nicotine produced by the castor bean plant and the tobacco plant, respectively (7,11,20,25,26). In plants, one metabolic pathway leading to the formation of pyridine nucleotides is the de novo pathway, involving initially the condensation of aspartic acid and glyceraldehyde-3-P or closely related metabolites. The condensation product of aspartic acid and glyceraldehyde-3-P undergoes a series of reactions leading to the formation of quinolinic acid which is then converted to pyridine nucleotides. The only known enzyme in this pathway is quinolinic acid phosphoribosyltransferase, which catalyzes the conversion of quinolinic acid and phosphoribosylpyrophosphate to nicotinic acid mononucleotide (7,15). The latter is then converted to NAD via nicotinic acid adenine dinucleotide (16). By means of the pyridine nucleotide cycle, the NAD so made in plants is eventually broken down into nicotinamide and nicotinic acid, which in a cyclic process can also be converted to nicotinic acid mononucleotide and subsequently to NAD (6,22). The known reactions of the de novo synthesis of pyridine nucleotides and the reactions of the pyridine nucleotide cycle are shown in Figure 1. Because quinolinic acid is easily converted to nicotinamide and nicotinic acid (7), and because all three compounds are excellent precursors of the pyridine alkaloids, ricinine and nicotine, the pyridine nucleotide cycle could lie between the de novo pathway for pyridine nucleotide biosynthesis and the pyridine alkaloids, as first s...

Nicotine Biosynthetic Enzyme Activities in Nicotiana tabacum L. Genotypes with Different Alkaloid Levels

“…Earlier speculation of the role of QPT in nicotine biosynthesis (12), also is supported by present data. The combination of greater activity in roots than leaves, increased activity level in response to plant decapitation, and proportionality to leaf nicotine levels provides strong evidence for the involvement of QPT in nicotine synthesis.…”

“…2) QPT is the only one of the three enzymes examined in the roots that is thought to be involved in both primary (pyridine nucleotide synthesis) and secondary (nicotine synthesis) metabolism (12). As such, there exists the possibility of both or only one function being associated with the alleles that condition low nicotine level.…”

“…For synthesis of the pyridine ring, quinolinic acid and nicotinic acid are efficient precursors (24). High levels of quinolinic acid phosphoribosyltransferase (EC 2.4.2.19) but not nicotinic acid phosphoribosyltransferase or nicotinamide deamidase in N. rustica roots, but not in leaves, suggests that QPT4 levels may be responsive to changing metabolic demands for the pyridine ring such as likely occurs during nicotine synthesis (12).…”

Crystalline Quinolinate Phosphoribosyltransferase fromAlcaligenes eutrophussubsp.quinolinicus: Effects of Metal Ions, Phosphorous Compounds, Niacin Nucleotides and Glycerol

“…In castor bean, the level of this enzyme changed according to the stage of development. 6 ) The effects of various substances on the enzyme activity were investigated using crystalline hog liver enzyme.…”

Determination of alkaloid compounds of Ricinus communis by using gas chromatography- mass spectroscopy (GC-MS)

“…It was first discovered and named by Tuson in the seeds of R. communis, while searching for its medically active compounds even before ricin was known. Subsequently, its chemical structure was identified and its biosynthesis and metabolism was studied (Waller and Skursky, 1972;Mann and Byerrum, 1974).…”