Metabolism and Excretion

The conversion of tryptophan to nicotinic acid in vivo is depicted in Figure 1. The rate of conversion of tryptophan to niacin and the pyridine nucleotides is controlled by the activities of trypto-phan dioxygenase (known alternatively as trypto-phan pyrrolase), kynurenine hydroxylase, and kynureninase. These enzymes are, in turn, dependent on factors such as other B vitamins, glucagon, glu-cocorticoid hormones, and estrogen metabolites, and there are various competing pathways which also affect the rate of conversion. For these reasons, a variety of nutrient deficiencies, toxins, genetic and metabolic abnormalities, etc. can influence niacin status and requirements.

For practical purposes, on the basis of studies performed in the 1950s, 60 mg tryptophan is deemed to give rise to 1 mg nicotinic acid; hence 60 mg tryptophan contributes 1 mg niacin equivalent, for dietary intake calculations and food tables (see Table 1).

The two pyridine nucleotide coenzymes, formerly known as 'coenzymes I and II,' then for a period as 'DPN and TPN,' and known nowadays as 'NAD' and 'NADP' (nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate), are involved in hundreds of enzyme-catalyzed redox reactions in vivo. Although a minority of these

Tryptophan

Tryptophan

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