Indoleamine 2,3-dioxygenase. Formation of L-kynurenine from L-tryptophan in cultured rabbit fineal gland

The distribution of the indoleamine 2,3-dioxygenase activity was investigated in various parts of the rabbit brain using the supernatant fraction (30,000 X g, 30 min) of homogenates. A low but significant activity was detected in all parts of the brain. The highest activity was associated with the pineal gland and choroid plexus. Specific activities of the supernatant fractions derived from the pineal gland and choroid plexus were 84.8 and 34.2 pmol/h/mg of protein at 37 degrees C, respectively, with L-tryptophan as substrate. When the pineal gland was cultured with L-[methylene-14C]tryptophan, L-[methylene-14C]kynurenine formed by the action of indoleamine 2,3-dioxygenase was found as one of the major products. It was isolated by DEAE-cellulose column chromatography and identified by thin layer chromatography with and without the treatment by kynureninase from a pseudomonad. The amount of kynurenine thus measured accounted for approximately one-third of the total amount of tryptophan metabolites, indicating that the kynurenine pathway is one of the major metabolic pathways of tryptophan in the rabbit pineal gland.