Decreased serum and red blood cell kynurenic acid levels in Alzheimer's disease.

Kynurenine aminotransferases (KAT I and KAT II) are responsible for the transamination of kynurenine (KYN) to form kynurenic acid (KYNA), an excitatory amino acid receptor antagonist. Since these members of the kynurenine pathway (KP) are proposed to be involved in the pathogenesis of Alzheimer's dementia (AD), the activities of these enzymes and the levels of these metabolites were measured in the plasma and red blood cells (RBCs) of AD and control subjects together with the inheritance of the apolipoprotein (APOE) epsilon4 allele. KYNA levels were significantly decreased both in the plasma and in the RBCs in AD, but the levels of KYN and the activities of KAT I and KAT II remained unchanged. No association has been found with the possession of the epsilon4 allele. These findings indicate an altered peripheral KP in AD regardless of the APOE status of the probands.