Presence of kynurenine hydroxylase in developing rat brain.

Kynurenine-3-hydroxylase, an enzyme that is part of the degradative pathway for tryptophan, was present in the cerebral cortex of neonatal rats and exhibited a Km for L-kynurenine close to that of the liver enzyme. This enzyme was enriched in mitochondrial fractions isolated from cerebral cortices of neonatal rats by Ficoll--sucrose gradient centrifugation, with some activity also present in synaptosomal fractions probably due to the mitochondrial content of synaptosomes since cytochrome c oxidase, another mitochondrial enzyme, had a similar distribution in the gradient. Kynurenine hydroxylase as well as monoamine oxidase, another mitochondrial enzyme, had increased specific activities in synaptosomal fractions isolated from 14-day-old rats compared to fractions from 8-day-old rats. Hypothyroidism, induced on the day of birth, resulted in increased activities of kynurenine hydroxylase and monoamine oxidase in synaptosomal fractions isolated from 14-day-old rats.