Abstract
Pyridine nucleotides not only act as mediators of redox reactions but also have important roles in signaling pathways and in gene regulation. The turnover of NAD(P) is performed by the pyridine nucleotide cycle (PNC). Diversity in PNC has been found in species and organs. In the present study, we examined details of the PNC in higher plants by investigating the metabolism of 14C-labelled compounds, by determining the enzyme activities of several plant materials, and by bioinformatic analyses. In plants, the specific PNC which consist of seven enzymes (PNCVII) is proposed. Proposed cycle is NAD → nicotinamide mononucleotide → nicotinamide riboside → nicotinamide → nicotinic acid → nicotic acid mononucleotide → NaAD → NAD. In addition, the cycle including nicotinic acid riboside is found in some plant species. Nicotinic acid, an intermediate of the cycle, is released for the synthesis of trigonelline and/or nicotinic acid glucoside.
