Nicotinate, also called nicotinic acid or niacin, is a form of vitamin B3 that is primarily obtained through whole and processed, as well as fortified foods. Another form of vitamin B3 is nicotinamide or niacinamide, which is also obtained in trace amounts from dietary sources. Nicotinamide is critically important in the structure of NAD(H) and NADP(H), which are both used as coenzymes in oxidation-reduction reactions such as the citric acid cycle and the electron transport chain. L-aspartic acid from the aspartate metabolism pathway can be converted into iminoaspartic acid by a putative L-aspartate dehydrogenase, removing a hydrogen ion from it. After this, a quinolinate synthase, whose protein is currently unknown in Xenopus laevis, converts it to quinolinic acid, which can also be obtained from the tryptophan metabolism pathway. Quinolinic acid can interact with carboxylating nicotinate-nculeotide pyrophosphorylase, which converts it to nicotinate beta-D-ribonucleotide. This can then be converted to nicotinate D-ribonucleoside by a cytosolic 5'-nucleotidase, adding a water molecule and removing a phosphate. This nicotinate D-ribonucleoside can then be converted back to nicotinate beta-D-ribonucleotide by a nicotinamide riboside kinase, or converted to and from nicotinic acid by purine nucleoside phosphorylase. Nicotinate beta-D-ribonucleotide can alternatively be converted straight to and from nicotinic acid by nicotinate phosphoribosyltransferase. Nicotinate beta-D-ribonucleotide can also be converted to and from nicotinic acid adenine dinucleotide by nicotinamide-nucleotide adenylyltransferase. This can then be converted back by a nucleotide diphosphatase. The nicotinic acid adenine dinucleotide is then converted to NAD by glutamine-dependent NAD synthetase. NAD can be converted to NADP by NAD kinase 2 in the mitochondria, which can be converted to and from NAD by a NAD+ transhydrogenase, also in the mitochondria. NAD can also be converted to nicotinamide ribotide by a nucleotide diphosphatase, which can then be converted to and from NAD by nicotinamide-nucleotide adenylyltransferase. Nicotinamide ribotide can form nicotinamide riboside via catalysis by a cytosolic purine 5'-nucleotidase, which can be returned to nicotinamide ribotide by a ribon. It can also be converted to nicotinamide by a purine nucleoside phosphorylase, which removes ribose 1-phosphate from it. Nicotinamide ribotide can also be directly converted to and from nicotinamide by nicotinamide phosphoribosyltransferase which removes phosphoribosyl pyrophosphate from it. Additionally, NADP and nicotinic acid can form nicotinamide and nicotinic acid adenine dinucleotide phosphate, catalyzed by 2'-phospho-cyclic-ADP-ribose transferase. Nicotinamide can finally have a methyl group added to it by nicotinamide N-methyltransferase, forming 1-methylnicotinamide, which can then interact with aldehyde oxidase 5, forming either N1-methyl-4-pyridone-3-carboxamide or N1-methyl-2-pyridone-5-carboxamide, which are the final products of this pathway.

Nicotinate and Nicotinamide Metabolism References