Abstract
IN the oxidation–reduction metabolism of a cell the pyridine nucleotide NAD is used catalytically; any NAD that is reduced is reoxidised. But NAD is consumed in certain metabolic reactions in which it serves as a substrate. In eukaryotic cells, the most intriguing of these reactions is the cleavage of NAD to form nicotinamide and a unique polymer, poly adenosine diphosphoribose (poly ADPR) (Fig. 1); the reaction is catalysed by the enzyme poly ADPR synthetase1–4. Other well studied reactions involving the destruction of NAD include the cleavage of NAD to form AMP and NMN by bacterial DNA ligases5,6 and the breakdown of NAD (with the concomitant inactivation of protein synthesis) by diphtheria toxin7,8.
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