Kynurenine metabolism in plasma and in red blood cells in Parkinson's disease.

Substantial evidence indicates that neuroactive kynurenine metabolites play a role in the normal physiology of the human brain, and are involved in the pathology of neurodegenerative disorders such as Parkinson's disease (PD). A sidearm product of the pathway, kynurenic acid (KYNA), which is synthesized by the irreversible transamination of kynurenine (KYN) by kynurenine aminotransferases (KAT I and KAT II), is an excitatory amino acid receptor antagonist. In the present study we measured the level of KYNA and the activities of the biosynthetic enzyme isoforms KAT I and KAT II in the plasma and in the erythrocytes (RBC) of 19 PD patients and 17 age-matched controls. The KAT I and KAT II activities were significantly lower in the plasma of PD patients, followed by a tendency to a decrease in plasma KYNA. An elevated KYNA level correlated with a significant increase in KAT II activity in the RBC of PD patients. These data support the contribution of an altered KYNA metabolism in the RBC to the pathogenesis of PD. The increased activity of KAT II in correlation with the elevated KYNA level in the RBC may mediate a consecutive protective response against excitatory neurotoxic effects.