The regulation of enzyme activities of the nicotine pathway in tobacco

The activities of the three enzymes of the route ornithine to the N-methyl-Δ¹-pyrrolinium salt and of the eight enzymes of the route quinolinic acid to nicotinic acid (pyridine nucleotide cycle) were determined in the roots of different Nicotiana tabacum L. cultivars and for comparison in tomato ( Lysopersicon esculentum L. cv. Vollendung) roots. Enzyme activities were further followed in different plant organs, dedifferentiated tissues, root organ cultures and in the roots after decapitation of tobacco plants. The data are in accord with the concept that the two routes are predominantly regulated by the enzymes putrescine methyltransferase (EC 2.1.1.53) and quinolinic acid phosphoribosyltransferase (EC 2.4.2.19), respectively. Further regulation involves the enzymes NAD⁺-pyrophosphatase (EC 3.6.1.22) and nicotinic acid mononucleotide glycohydrolase; these findings confirm the suggestion that the transformation to nicotinic acid is performed along two routes: either via the synthesis and degradation of NAD⁺ or by a direct step through the action of a specific glycohydrolase.