Oxygen affinity of tyrosine and tryptophan hydroxylases in synaptosomes

Tyrosine and tryptophan hydroxylase activities were studied in a synaptosome-enriched (P2) preparation from the rat striatum. Care was taken to avoid the potential diffusional artifacts to which measures of oxygenase activities in respiring tissue are, in general, subject. Tyrosine hydroxylase exhibited a Km for oxygen of 2 to 3 mm Hg under a variety of conditions. Tryptophan hydroxylase exhibited a Km of 3 to 4 mm Hg at C.S.F. levels of tryptophan (10 microM). The Km increased to 8 to 10 mm Hg at 2 microM tryptophan. These values are all consistent with some degree of unsaturation with respect to oxygen in vivo.