Glutamate, AMP, PP; Fig. 4.2. The biosynthetic steps in converting niacin to NAD.
intracellular NAD levels. The liver is the central processing organ for niacin. The liver and kidneys can convert tryptophan to quinolinic acid, which reacts with phosphoribosyl pyrophosphate (PRPP) to form NaMN as catalysed by quinolinate phosphoribosyltransferase in these tissues (Moat and Foster, 1982).
There are two major roles played by NAD. Together with NADP, the pyridine nucleotides are redox coenzymes that participate as hydride ion trafficking agents in numerous oxidation-reduction reactions. Most NAD within the cell is oxidized, whereas most NADP is maintained in the reduced state as a result of the pentose phosphate pathway. Diverse enzymes utilizing either NAD or NADP are localized within cytosol and organelles. The other major role of NAD is as a substrate for enzyme systems, especially those that are membrane associated, that release nicotinamide and ADP-ribose as the mono-, poly- or cyclic product. The ADP-ribosylations affect diverse systems that include amino acid residues on acceptor proteins. Transferase proteins are found in nuclear, cytosolic and plasma membrane fractions. An NAD: arginine ADP-ribosyltransferase is anchored via glycosylphosphatidylinositol to the outer surface of the plasma membrane (Zolkiewska et al., 1994). Poly(ADP-ribose) is synthesized by a polymerase that affects DNA repair. The cyclic ADP-ribose affects calcium ion transport. Also a deamidated metabolite of NADP (NAADP) is a potent Ca2+-releasing agent distinct from the cyclic ADP-ribose (Lee, 2000).
Liver has some capacity for NAD storage (Kirkland and Rawling, 2001). It is also the organ that forms most of the major methylated and hydroxylated catabolites excreted in urine. In humans, nicotinamide is methylated primarily to produce .M-methylnicotinamide though some is oxidized to yield .1-methyl-2-pyridone-5-carboxamide. Nicotinamide is conjugated mainly to glycine to form nicotinuric acid.
The vitamin B6 group is comprised of the three vitamers, i.e. pyridoxine (PN), pyridoxamine (PM) and pyridoxal (PL). In addition, there are three 5'-phosphates of these, i.e. PNP, PMP and PLP. The predominant coenzyme is PLP, which is also the major form in most natural foods. The 5'-phosphates are hydrolysed by alkaline phosphatase in the small intestinal lumen (Henderson, 1985). As with most other water-soluble vitamins, at physiological concentrations the absorption is facilitated with metabolic trapping. Upon entry into enterocytes and other cells, all three vitamers are phosphorylated (McCormick, 2001). Liver is the primary organ responsible for much of the metabolism of vitamin B6 and its subsequent distribution, much of it as albumin-bound PLP. Cytosolic interconversions within the B6 group require phosphorylation of the vitamers catalysed by pyridoxal phosphokinase, optimal with Zn2+ATP in eukaryotes (McCormick et al., 1961), and pyridoxine (pyridoxamine) 5'-phosphate oxidase (Kazarinoff and McCormick, 1975), which, because of its FMN dependency, is sensitive to riboflavin status (McCormick, 1989). These activities have been measured in human liver (Merrill et al, 1984) where the PLP formed is trapped within the cells not only by the anionic charge on the phosphate moiety but also by reaction of its aldehyde function with internal proteins, resulting in reversible Schiff bases (Li et al., 1974). Release of PLP from cells is mainly after hydrolysis catalysed by phosphatase bound to the plasma membrane (Merrill and Henderson, 1990). Distribution of B6 and its metabolites is widespread in tissues (McCormick, 2001), but much is in skeletal muscle as PLP (Coburn et al., 1991), a considerable fraction associated with phosphorylase (Black et al., 1978). It is noteworthy that brain, rich in pyridoxal kinase (McCormick and Snell, 1959), requires PLP in the metabolism of glutamate and formation of amines active in the nervous system.
The enzymes that require PLP are found in the cytosol and organelles. Formation of coenzyme is in the former as is its attachment to apoenzymes, some of which remain cytosolic while some are transferred to specific organelles. Intracellular targeting of PLP-dependent enzymes commonly involves a sequence motif that 'directs' the ultimate location of the holoenzyme. With the pre-mitochondrial form of aspartate aminotrans-ferase, there is an N-terminal sequence that interacts with a heat shock protein 70 (Hsp70) chaperone to guide the enzyme into the mitochondria (Artigues et al., 2000), whereas the cytosolic form of the enzyme lacks this motif. With the human ornithine decarboxylase that goes to the plasma membrane, a phosphorylation-regulated, p47phox-related, membrane-targeting motif surrounds Ser167 (Heiskala et al., 2000). As far as concerns non-enzymatic functions of PLP, reports that it may be involved in steroid hormone regulation and gene expression have been reviewed (Leklem, 2001).
The catabolism of B6 in the human primarily is the oxidation of pyridoxal to 4-pyridoxic acid catalysed by aldehyde oxidase. The 4-pyridoxate is an end-product that is excreted in urine.
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