N

HO OR HO OH

Reduced form (NADH, NADPH)

R = H Nicotinamide adenine dinucleotide R = PO32- Nicotinamide adenine dinucleotide phosphate Figure 28.28 • Structures of niacin-derived cofactors.

food is treated in a manner that releases the niacin. For example, some grains (i.e., corn) may contain predominantly niacin esterified to polysaccharides or peptides, referred to as niacytin. This form has a relatively low bioavailability unless treated with alkali in cooking or processing. Coffee beans have a high niacin content in the form of trigonellin (1-methyl nicotinic acid) that is converted to niacin upon roasting. Most dietary niacin from natural sources is in the form of the nucleotide derivatives, NAD+ and NADP+, which are digested by NAD + diphosphatase (EC 3.6.1.22 ), followed by hydrolases in the intestine to release nicotinamide. Free nicotinamide and niacin are absorbed through a facilitated diffusion process in the intestine.156 At physiological doses, little niacin is excreted unchanged. Most is excreted as N-methylniacin or the glycine conjugate (nicotinuric acid). After administration of large doses, niacin can be found in the urine unchanged.

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