Kynurenine 3-Hydroxylase in Brain: Species Activity Differences and Effect of Gerbil Cerebral Ischemia

We have developed a rapid and highly sensitive assay for brain kynurenine 3-hydroxylase activity. The present study determined some characteristics and species differences of kynurenine 3-hydroxylase activity in brain and evaluated the response of this enzyme to cerebral ischemia. The kynurenine 3-hydroxylase assay is based on the conversion of L-kynurenine to 3-hydroxykynurenine in vitro and the quantification of 3-hydroxykynurenine by high-performance liquid chromatography. Kynurenine 3-hydroxylase activity was detected in human, macaque, rat, mouse, and gerbil brain. Regional gerbil brain activities ranged from 20 to 50 nmol/g/h, while kynurenine 3-hydroxylase activities in other species were one order of magnitude lower. Kynurenine 3-hydroxylase was also detected in lung, kidney, spleen, intestine, and liver of gerbils, with activities larger than in brain. Delayed increases in the activity of kynurenine 3-hydroxylase occur in several brain regions following transient ischemia in gerbils. These changes are particularly marked in regions that showed the most extensive brain damage (hippocampus and striatum). Kynurenine 3-hydroxylase may have an important role in determining the flux of kynurenine in brain.