One of the biologically active forms of nicotinic acid. Serves as a coenzyme of hydrogenases and dehydrogenases. NAD usually acts as a hydrogen acceptor, forming NADH which then serves as a hydrogen d
onor in the respiratory chain. Present in living cells primarily in the reduced form (NADPH) and is involved in synthetic reactions.
Occurs in 2 forms, α-NAD and β-NAD, distinguished by the configura
tion of the ribosyl nicotinamide linkage. Only the β-anomer is bioactive.
This coenzyme is available in high purity, and it is advisable to buy a fresh preparation rather than to purify an old sample as purification will invariably lead to a more impure sample contaminated with the oxidised form (NAD). It has UV max at 340nm ( 6,200 M-1cm-1) at which wavelength the oxidised form NAD has no absorption. At 340nm a 0.161mM solution in a 1cm (pathlength) cell has an absorbance of 1.0 unit. The purity is best checked by the ratio A280nm/A340nm ~2.1, a value which increases as oxidation proceeds. The dry powder is stable indefinitely at -20o. Solutions in aqueous buffers at pH ~7 are stable for extended periods at -20o and for at least 8hours at 0o, but are oxidised more rapidly at 4o in a cold room (e.g. almost completely oxidised overnight at 4o). [UV: Drabkin J Biol Chem 175 563 1945, Fluorescence: Boyer & Thorell Acta Chem Scand 10 447 1956, Redox: Rodkey J Biol Chem 234 188 1959, Schlenk in The Enzymes 2 250, 268 1951, Kaplan in The Enzymes 3 105, 112 1960.] Deuterated NADH, i.e. NADD, has been purified through the anion exchange resin AG-1 x 8 (100-200 mesh, formate form) and through a Bio-Gel P-2 column. [Viola et al. Anal Biochem 96 334 1979, Beilstein 26 III/IV 3639.]