Studies on the Oxygen Sensitivity of Tryptophan Hydroxylase

Abstract

Tryptophan hydroxylase [EC 1.14.16.4; 1- tryptophan, tetrahydropterin: oxygen oxidoreductase (5-hydroxylating)] catalyzes the initial and rate limiting step in the biosynthesis of the neurotransmitter serotonin (Jequier et al, 1967). Since the discovery of this enzyme in extracts of brain tissue (Lovenberg et al, 1967; Gal et al, 1966, Grahame-Smith, 1964) relatively little progress has been made in delineating its physical properties and kinetic mechanisms, apparently because of the extreme lability of the enzyme to standard purification procedures. For example, the most highly purified (85–90% pure) preparation of brain tryptophan hydroxylase (Tong and Kaufman, 1975) is scarcely 2 to 3 times more active than the crude enzyme (see Levine et al, 1979). Since a number of recent studies have reported only moderate success in the purification of tryptophan hydroxylase from a variety of tissues (Youdim et al, 1975; Widmer et al, 1975), we decided to determine which factors are most important in contributing to the liability of this enzyme. The studies of Hori and colleagues (Hori, 1975; Hori et al, 1976; Hori and Ohotani, 1978; Ichiyama et al, 1974) on the activation of bovine pineal tryptophan hydroxylase by dithiothreitol-Fe²⁺ were perhaps the most influential in suggesting to us that the hydroxylase could be an oxygen sensitive enzyme.