Kynureninase: synthesis of substrate analogues and mechanistic studies

Abstract

Kynureninase (EC.3.7.1.3) is an unusual pyridoxal 5'-phosphate (PLP) dependent enzyme which catalyses the beta-gamma hydrolytic cleavage of kynurenine to give anthranilic acid and alanine. The enzyme is a potential therapeutic target for controlling the levels of metabolites, quinolinic acid and kynurenic acid, that are important in a variety of neurodegenerative and inflammatory disorders, such as Alzheimer's disease, AIDS, Lyme disease and poliovirus. Studies have been carried out to elucidate the mechanism of the enzyme. A phosphinic acid analogue of kynurenine, designed as a putative transition state mimic for the reaction, has been prepared. This was found to be a competitive inhibitor, with a KI of 4.19 mM when its interaction with kynureninase, isolated from Pseudomonas fluorescens, was examined. The corresponding methyl phosphinate was also prepared. This was found to be a more potent inhibitor, with a KI of 0.88 mM. It is proposed that methylation of the acid removes a destabilising interaction between the negative charge of the ionised phosphinic acid and some group of the active site. A number of analogues of kynurenine were prepared in order to obtain information on the specificity of the enzyme and the interactions at the active site. A racemic mixture of desaminokynurenine was found to have a KI of 23.2 µM. Cyclohexyl and naphthyl amino acid derivatives prepared by the same method were found to be weak competitive inhibitors of the enzyme with KI values of 844 µM and 207 µM repectively. Desaminokynurenine was found to be a substrate with a rate of reaction twenty times slower than that of kynurenine. The naphthyl amino acid derivative was also found to be a substrate. The results suggested that interactions with the aromatic ring are important and that the naphthyl amino acid derivative may be too large for the active site. The synthesis of 25-[2-2H]-kynurenine is described using two different routes. Diacetylation / racemisation of racemic kynurenine in deuterium oxide followed by acylase catalysed resolution is the most direct route. The alternative is to prepare 2S-[2-2H]-tryptophan by a similar procedure and then to convert this to 2S-[2-2H]-kynurenine via ozonolysis. This compound has been used in isotope studies to gain an understanding of the mechanism of the enzyme catalysed reaction. The primary deuterium isotope effect, DV= 0.98 and D(V/K) = 3.6, showed that for the kynureninase catalysed reaction, alpha-H abstraction is partially rate limiting. A solvent isotope effect, DV= 4.4 and D(V/K) = 4.6 was also determined, showing that a proton transfer is also partially rate limiting.