Ch

Nicotinate (niacin), a precursor of NAD and NADP

CH2COO"

Indoleacetate, a plant growth factor

Indoleacetate, a plant growth factor

FIGURE 18-22 Tryptophan as precursor. The aromatic rings of tryptophan give rise to nicotinate, indoleacetate, and serotonin. Colored atoms trace the source of the ring atoms in nicotinate.

Phenylalanine Catabolism Is Genetically Defective in Some People

Given that many amino acids are either neuro-transmitters or precursors or antagonists of neutrotransmitters, genetic defects of amino acid metabolism can cause defective neural development and mental retardation. In most such diseases specific intermediates accumulate. For example, a genetic defect in phenylalanine hydroxylase, the first enzyme in the catabolic pathway for phenylalanine (Fig. 18-23), is responsible for the disease phenylketonuria (PKU), the most common cause of elevated levels of phenylalanine (hyperphenylalaninemia).

Phenylalanine hydroxylase (also called phenylala-nine-4-monooxygenase) is one of a general class of enzymes called mixed-function oxidases (see Box 21-1), all of which catalyze simultaneous hydroxylation of a substrate by an oxygen atom of O2 and reduction of the other oxygen atom to H2O. Phenylalanine hydroxylase

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