Species Differences inl-Tryptophan–Kynurenine Pathway Metabolism: Quantification of Anthranilic Acid and Its Related Enzymes

Abstract

Anthranilic acid (AA) has attracted considerable attention as one of thel-tryptophan–kynurenine pathway metabolites in the central nervous system. In this study, a highly sensitive and accurate method for the quantification of AA has been developed using reversed-phase high-performance liquid chromatography with electrochemical detection. Serum and cerebrospinal fluid (CSF) AA concentrations in different animal species were measured. CSF AA concentrations in rabbits were 1.1 ± 0.1 nmol/liter, which were 5.7–33.0 times lower than those in other species studied. Serum AA concentrations, however, were slightly higher in rabbits than in other species. In contrast, the concentrations ofl-kynurenine (L-KYN) in both serum and CSF were substantially higher in rabbits than in other species. Tissue kynurenine pathway enzymes, indoleamine 2,3-dioxygenase (IDO), tryptophan 2,3-dioxygenase, kynurenine 3-hydroxylase, and kynureninase were determined in rabbits, rats, gerbils, and mice. These enzymes varied among species, especially lung IDO activities in rabbits were 146–516 times higher than those found in other species, but rabbit liver kynurenine 3-hydroxylase activities were lower by one order of magnitude than those of the other species tested. Furthermore, brain kynurenine 3-hydroxylasae activities were 12.3–23.2 times higher in gerbils than those in the other species tested. In addition, AA concentrations in serum following intravenous administration of L-KYN (5 mg/kg) were also measured in rabbits. AA levels peaked sharply within 5 min after administration and decreased in a time-dependent manner. At 5 min after administration, CSF L-KYN and AA concentrations were also increased by 1.76- and 2.56-fold, respectively, compared with basal levels. Increased AA concentrations in CSF following L-KYN administration may reflect the entry of AA into the CSF after conversion to AA in systemic tissue and/or the local synthesis of AA from L-KYN in the CNS.