Transport of Kynurenic Acid by Rat Organic Anion Transporters rOAT1 and rOAT3: Species Difference between Human and Rat in OAT1

Abstract

A tryptophan catabolite, kynurenic acid, is involved in schizophrenia and uremia; there is little information on the mechanism of its disposition. Recently, our laboratory showed that kynurenic acid is a good substrate of human organic anion transporters hOAT1 and hOAT3. In this study, we performed uptake experiment using Xenopus laevis oocytes to characterize the transport of kynurenic acid by rat homologs of the transporters, rOAT1, and rOAT3. These transporters stimulated the uptake of kynurenic acid into oocytes, and transport by rOAT3 was marked. The Km values of the transport were estimated to be 8.46 μM for rOAT1 and 4.81 μM for rOAT3, and these values are comparable to their human homologs. The transport activity of kynurenic acid by rOAT1 was about one quarter of that of p-aminohippurate, although they were at the similar levels in hOAT1. A comparative experiment with hOAT1 was added in this study, showing that uptake amounts of kynurenic acid by hOAT1-expressing oocytes were 4 times greater than rOAT1-expressing oocytes. rOAT3 transported kynurenic acid as efficiently as estrone sulfate; this phenomenon was also observed in hOAT3. In conclusion, transport of kynurenic acid by rOAT1 and rOAT3 was shown. The characteristics of rOAT3 were similar to hOAT3, but low transport activity of kynurenic acid by rOAT1 was exhibited compared with hOAT1.

Transport of Xanthurenic Acid by Rat/Human Organic Anion Transporters OAT1 and OAT3

“…6,13) The present study confirms that its derivative, xanthurenic acid, is also their substrate. Of the kynurenine pathway metabolites, xanthurenic acid is the second compound that was showed to be transported by OAT1 and OAT3.…”

“…13) Because no compound that showed species difference in transport by OAT1 had been identified, this finding was surprising. As shown in Fig.…”

“…This phenomenon was also observed in our previous study. 13) In xanthurenic acid, hOAT1 showed more uptake than rOAT1. There was no significant difference between rOAT1 and hOAT1 in the uptake of p-aminohippurate, their typical substrate, suggesting that hOAT1 transported xanthurenic acid as well as kynurenic acid more efficiently than rOAT1.…”

“…An uptake experiment using Xenopus laevis oocytes was performed as previously reported. 6,13) Briefly, capped RNA encoding each of the various organic anion transporters was transcribed from appropriate restriction enzyme-linearized pBK-CMV containing cDNA of rOAT1, hOAT1, rOAT3, or hOAT3, with T3 RNA polymerase. After 50 nL of water or cRNA (25 ng) was injected into defolliculated oocytes, the oocytes were maintained in modified Barth's medium (88 mM NaCl, 1 mM KCl, 0.33 mM Ca(NO 3 ) 2 , 0.4 mM CaCl 2 , 0.8 mM MgSO 4 , 2.4 mM NaHCO 3 , and 5 mM HEPES, pH 7.4) containing 50 mg/L gentamicin at 18 C. Two days after injection, the uptake reaction was initiated by incubating the oocytes in 500 mL uptake buffer (96 mM NaCl, 2 mM KCl, 1.8 mM CaCl 2 , 1 mM MgCl 2 , and 5 mM HEPES, pH 7.4) with each y To whom correspondence should be addressed.…”

“…Lower transport activity of kynurenic acid by rOAT1 was confirmed, in comparison to hOAT1. 13) Because their amino acid sequences show 87.8% identity, interaction between kynurenic acid and OAT family is interesting. 14) In the present study, to examine the transport of xanthurenic acid by rOAT1, hOAT1, rOAT3, and hOAT3, we conducted uptake experiments by the Xenopus laevis oocyte expression system.…”