Endogenous sources

L-Tryptophan (Trp). both from foods and from tissue protein breakdown, is an important alternative for preformed niacin. A small proportion of the available tryptophan is metabolized,

A small proportion ofcatabolizedTrp (as well as tryptamine, 5-hydroxytryptophaii, serotonin and melatonin) gives rise to nicotinamide-containing compounds (Magrii etal., 1999). Impaired absorption in Hartnup syndrome is associated with pellagra-like skin changes that are typical for niacin deficiency. The branch-point is at the conversion of 2-amino-3-carboxymuconate semialdehvde. While most of this intermediate is enzymatically deearboxylated a small proportion undergoes spontaneous dehydration

OH OH

Nicotinamide adenine dinucleotide (NAD)

-NH,

\

N

*

O

/ X

OH OH

Nicotinamide adenine dinucleotide phosphate (NADP)

Figure 10,17 Niacin-derived ¿¡nucleotides to quinolinic acid (pyridine-2.3-dicarboxy late I, The ratio of NAD derived from tryptophan to total metabolized tryptophan is usually assumed to be about 1:60 (Horwitt et ul., 19811. Nicotinate-nucleotide pyrophosphoryluse (EC2.4.2.19) decarboxylases quinolinate and attaches a ribose phosphate moiety. Magnesium-dependent nicotinate-nucleotide adenyly I transferase (EC2.7.7.18, Schweîger et al.. 2001) adds adenosine phosphate and amination by either of two NAD synthases (EC6.3.5 1 and ECft.3.1.5) then completes synthesis of NAD (nicotinamide adenine dinucleotide) Breakdown of NAD can generate both nicotinamide and nicotinate.

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