l-Kynurenine-3-hydroxylase was solubilized from the outer membrane fraction of rat liver mitochondria, and the enzyme was partially purified by ammonium sulfate fractionation. The kynurenine hydroxylase was independent of the cytochrome b5 reductase system which was also localized in the outer membrane fraction. β-NADH was found to be effective as an electron donor for the partially purified kynurenine hydroxylase, as was NADPH. Under conditions in which whole mitochondria were used as enzyme sample, β-NADH seemed not to act as electron donor for the kynurenine hydroxylase, presumably because of diversion of the electron flow to the respiratory chain in mitochondrial inner membrane. When the electron transfer in the respiratory chain was inhibited by rotenone, antimycin A, or cyanide, β-NADH became available as electron donor for the kynurenine hydroxylase.
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