# Kynurenic Acid Synthesis from D-Kynurenine in the Cerebellum: A Distinct Role of D-Amino Acid Oxidase

## TL;DR

This study shows that D-amino acid oxidase (D-AAO) plays a major role in producing kynurenic acid (KYNA) in the cerebellum, a brain region.

## Contribution

The study identifies D-AAO as a key enzyme for KYNA synthesis from D-KYN in the cerebellum, distinct from the traditional KATII pathway.

## Key findings

- D-AAO efficiently converts D-KYN to KYNA in vitro and in cerebellar homogenates.
- Inhibiting D-AAO significantly reduces KYNA production in the cerebellum, unlike KATII inhibition.
- In vivo experiments confirm that D-KYN perfusion increases extracellular KYNA levels, reduced by D-AAO inhibitors.

## Abstract

The enzymatic formation of kynurenic acid (KYNA), a neuromodulator metabolite of the kynurenine pathway (KP) of tryptophan metabolism, in the mammalian brain is widely attributed to kynurenine aminotransferase II (KATII). However, an alternative biosynthetic route, involving the conversion of D-kynurenine (D-KYN) to KYNA by D-amino acid oxidase (D-AAO), may play a role as well. In the present study, we first confirmed that purified D-AAO efficiently converted D-KYN—but not L-KYN—to KYNA. We then examined KYNA formation from D-KYN (100 µM) in vitro, using tissue homogenates from several human brain regions. KYNA was generated in all areas, with D-AAO-specific production being most effective by far in the cerebellum. Next tested in homogenates from rat cerebellum, KYNA neosynthesis was significantly reduced by D-AAO inhibition, whereas KATII inhibition had no effect. Finally, KYNA production was assessed by in vivo microdialysis in rat cerebellum. Local D-KYN perfusion, alone and in combination with inhibitors of D-AAO (kojic acid) or aminotransferases (AOAA), caused a substantive increase in extracellular KYNA levels. This effect was attenuated dose-dependently by micromolar concentrations of kojic acid, whereas co-perfusion of AOAA (1 mM) was ineffective. Together, our findings indicate that D-AAO should be considered a major contributor to KYNA production in the cerebellum, highlighting region-specific qualitative differences in cerebral KYNA metabolism.