NAD and NADP systems

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

Until now, only a few versatile, selective and effective transition-metal complexes have been applied in nicotinamide cofactor reduction. The TOFs are well within the same order of magnitude for all systems studied, and are within the same range as reported for the hydrogenase enzyme thus, the catalytic efficiency is comparable. The most versatile complex Cp Rh(bpy) (9) stands out due to its acceptance of NAD+ and NADP+, acceptance of various redox equivalents (formate, hydrogen and electrons), and its high selectivity towards enzymatically active 1,4-NAD(P)H. 

Ochoa reported that malic enzyme from L. plantarum was NAD and not NADP specific. The malic enzyme of cauliflower bud mitochondria (31) is NAD and NADP specific, with NAD being the preferred cofactor. Both the malo-lactic activity and NADH producing activity of the Leuconostoc oenos system (6,7, 8) was strictly NAD specific. Nicotinamide-adenine dinucleotide phosphate, flavin adenine dinucleotide, and flavin mononucleotide could not substitute in either of these activities. 

Why are there two pyridine nucleotides, NAD+ and NADP+, differing only in the presence or absence of an extra phosphate group One important answer is that they are members of two different oxidation-reduction systems, both based on nicotinamide but functionally independent. The experimentally measured ratio [NAD+] / [NADH] is much higher than the ratio [NADP+] / [NADPH]. Thus, these two coenzyme systems also can operate within a cell at different redox potentials. A related generalization that holds much of the time is that NAD+ is usually involved in pathways of catabolism, where it functions as an oxidant, while NADPH is more often used as a reducing agent in biosynthetic processes. See Chapter 17, Section I for further discussion. 

The processes of electroreduction, and subsequent photochemical regeneration of the parent monomers, consequently form a closed cycle involving the transport of electrons and protons, as occurs in the reduction and oxidation of other systems, e.g. coenzymes such as NAD+ and NADP+ 13,U). 

As pointed out by Krebs and Veech (176), the relationship between the redox states of NAD and NADP in mammalian cells would be governed to a large extent by the substrate levels of NAD- and NADP-dependent dehydrogenases, interlinked by shared reactants. The coordination of these systems of interlinked dehydrogenases and, in particular, energy-linked transhydrogenase has been a matter of controversy. Funda-... 

Structurally, NADP differs from NAD only by a phosphate group esterified at the 2 C of the adenosine ribose, a difference which is reflected in the enzymatic roles NAD-dependent dehydrogenases are mostly involved in catabolic reactions, while NADP-specific enzymes are usually confined to biosynthetic pathways (1). The marked specificities displayed by dehydrogenases towards NAD and NADP have provided attractive model systems to understand the process of molecular recognition by protein engineering. 

The two commercial forms of the vitamin, niacin and niacinamide, are rapidly absorbed from both the stomach and intestine. As the dose increases, absorption decreases. It is not clear whether there is a feedback mechanism operating or the transport system becomes saturated. Conversion to the coenzyme forms occurs in the cells where NAD and NADP are needed. 

The behavior of invertebrate and plant GDH s has been less extensively studied than that of the bovine enzyme. The question of compulsory order as opposed to random order binding, which has been resolved only with great difficulty for bovine GDH, has been investigated with only a few other GDH s. In each case, for Phycomycetes GDH (NAD) (S3S), GDH (NADP) of Brevibacterium flavum 30), soybean GDH (NAD), 7), and both the NAD- and NADP-dependent GDH s of Thio-bacillus novellm (33), compulsory order binding has been reported in which coenzyme binds first and NH4+ last. However, since the more refined methods employed for investigation of the mechanism of bovine GDH have not been applied to any of these systems, the question of random vs. ordered mechanism cannot be said to have been resolved, particularly since the methods thus far employed did not give decisive results with bovine GDH. 

Succinate is the electron donor (so it is oxidized) and FAD is the electron acceptor (so it is reduced). The products of the reaction are fumarate (oxidized) and FADH2 (reduced). Cells typically use a common set of electron acceptors, such as FAD, NAD+ and NADP+. Many of these molecules deposit their electrons into the electron transport system. 

One study deals with the putative conversion anthranilic acid - HA [32], in which the production of radiolabeled NAD and NADP was observed in rat livers in response to administration of radiolabeled anthranilic acid. In the experiments with the hepatic microsomal system, a mixture of 5-hydroxyanthranilic acid and HA was obtained. 

Mitochondrial ADP-ribosylation as described by Kun et al. [2,4] involves the apparent transfer of ADPR residues from NAD to a 50 kD polypeptide. Yet another ADP-ribosylation reaction was reported by Richter et al. [13,14], They described the modification of a 30 kD polypeptide insubmitochondrial particles when incubated with labeled NAD. This reaction was brought into connection with Ca efflux from mitochondria as induced by treatment of mitochondria with organic peroxides. Such treatment was known to result in the concomitant degradation of NAD and NADP to ADPR and P-ADPR, respectively [15]. During our attempts to characterize these mitochondrial systems, evidence accumulated that nonenzymic ADP-ribosylation is involved. The following observations support this interpretation ... 

Analysis of nicotinic acid, nicotinamide, and its metabolites in biological materials, i.e., blood, plasma, urine, and tissues, is important in studies on biochemical pathways (Hengen and deVries, 1985). Finder et al. (1971) described several paper and thin-layer chromatographic systems useful for the differentiation of nucleotides in tissues derived from nicotinamide and nicotinic acid. Hengen and deVries (1985) provided a table summarizing the Rp values of nicotinic acid, nicotinamide, and various intermediates of NAD+ and NADP-I- synthesis for both paper and thin-layer chromatography. Haworth and Walmsley (1972) used two-dimensional TLC on silica gel for the identification of tryptophan metabolites in urine and resolved 32 compounds including nicotinic acid and nicotinamide. Kala et al. (1978) used silica gel TLC to examine urine for nicotinic acid and its metabolites after administration of nicotinyl alcohol. They... 

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