Oxygen Affinity of Tyrosine and Tryptophan Hydroxylases in Synaptosomes

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Abstract
Tyrosine and tryptophan hydroxylase activities were studied in a synaptosome-enriched (P2) preparation from the rat striatum. Potential diffusional artifacts to which measures of oxygenase activities in respiring tissue are subject were avoided. Tyrosine hydroxylase exhibited a Km for O2 of 2-3 mm Hg under a variety of conditions. Tryptophan hydroxylase exhibited a Km of 3-4 mm Hg at CSF levels of tryptophan (10 .mu.M). The Km increased to 8-10 mm Hg at 2 .mu.M tryptophan. These values are all consistent with some degree of unsaturation with respect to O2 in vivo. [Characterization of the enzymes'' O2 dependence may aid in the development of a model describing the pathogenesis of cerebral hypoxia.].