In addition to being a source of serotonin, tryptophan uniquely serves as a precursor of a B-complex vitamin, nicotinamide or niacin (Fig. 4.12). However, the relatively low efficiency of this pathway means that for all practical purposes an exogenous supply of the vitamin is necessary to satisfy metabolic demands.
The hormone thyroxine is synthesized from tyrosine. It is, effectively, a dimer composed of two iodinated residues of tyrosine. Adrenaline is formed by methylation of noradrenaline (Fig. 4.10) by a reaction that is dependent on the participation of S-adenosylmethionine.
Tryptophan i 1
Nicotinamide Fig 4.12. Biosynthesis of nicotinamide from tryptophan. Enzymes: 1, kynurenine 3-oxygenase; 2, kynureninase; 3, quinolinate transphosphoribosyl 3s6.
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