Selective and competitive inhibition of kynurenine aminotransferase 2 by glycyrrhizic acid and its analogues

Yukihiro Yoshida,Akifumi Oda,Kuniaki Saito,Suwako Fujigaki,Yasuko Yamamoto,Akihiro Mouri,Hidetsugu Fujigaki,Toshitaka Nabeshima,Koichi Kato,Kazuo Kunisawa,Kyoka Yamazaki

doi:10.1038/s41598-019-46666-y

Yukihiro Yoshida, Akifumi Oda + Show 9 more

Open Access

https://doi.org/10.1038/s41598-019-46666-y

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Abstract

The enzyme kynurenine aminotransferase (KAT) catalyses the conversion of kynurenine (KYN) to kynurenic acid (KYNA). Although the isozymes KAT1–4 have been identified, KYNA is mainly produced by KAT2 in brain tissues. KNYA is an antagonist of N-methyl-D-aspartate and α-7-nicotinic acetylcholine receptors, and accumulation of KYNA in the brain has been associated with the pathology of schizophrenia. Therefore, KAT2 could be exploited as a therapeutic target for the management of schizophrenia. Although currently available KAT2 inhibitors irreversibly bind to pyridoxal 5′-phosphate (PLP), inhibition via this mechanism may cause adverse side effects because of the presence of other PLP-dependent enzymes. Therefore, we identified novel selective KAT2 inhibitors by screening approximately 13,000 molecules. Among these, glycyrrhizic acid (GL) and its analogues, glycyrrhetinic acid (GA) and carbenoxolone (CBX), were identified as KAT2 inhibitors. These compounds were highly selective for KAT2 and competed with its substrate KYN, but had no effects on the other 3 KAT isozymes. Furthermore, we demonstrated that in complex structures that were predicted in docking calculations, GL, GA and CBX were located on the same surface as the aromatic ring of KYN. These results indicate that GL and its analogues are highly selective and competitive inhibitors of KAT2.

Highlights

Dietary tryptophan is predominantly metabolised via the kynurenine pathway[1] (Fig. 1), which produces neuroactive metabolites such as kynurenic acid (KYNA)
Km values for glycyrrhizic acid (GL), glycyrrhetinic acid (GA) and CBX changed with substrate concentrations, whereas respective Vmax values did not vary under these conditions (Fig. 4b–d)
These results showed that GL, GA and CBX do not bind pyridoxal 5′-phosphate (PLP), but act as competitive inhibitors of KYN

Summary

Introduction

Dietary tryptophan is predominantly metabolised via the kynurenine pathway[1] (Fig. 1), which produces neuroactive metabolites such as kynurenic acid (KYNA). GL (IC50, 4.51 ± 0.20 μM; Ki, 10.42 ± 1.62 μM), GA (IC50, 6.96 ± 0.37 μM; Ki, 6.92 ± 0.60 μM) and CBX (IC50, 3.90 ± 0.37 μM; Ki, 4.11 ± 0.37 μM) showed high inhibitory activity against human KAT2 (Figs 3 and 4).

Results

Conclusion