Organic anion transporters, OAT1 and OAT3, are crucial biopterin transporters involved in bodily distribution of tetrahydrobiopterin and exclusion of its excess

Tetrahydrobiopterin (BH(4)) is a common coenzyme of phenylalanine-, tyrosine-, and tryptophan hydroxylases, alkylglycerol monooxygenase, and NO synthases (NOS). Synthetic BH(4) is used medicinally for BH(4)-responsive phenylketonuria and inherited BH(4) deficiency. BH(4) supplementation has also drawn attention as a therapy for various NOS-related cardio-vascular diseases, but its use has met with limited success in decreasing BH(2), the oxidized form of BH(4). An increase in the BH(2)/BH(4) ratio leads to NOS dysfunction. Previous studies revealed that BH(4) supplementation caused a rapid urinary loss of BH(4) accompanied by an increase in the blood BH(2)/BH(4) ratio and an involvement of probenecid-sensitive but unknown transporters was strongly suggested in these processes. Here we show that OAT1 and OAT3 enabled cells to take up BP (BH(4) and/or BH(2)) in a probenecid-sensitive manner using rat kidney slices and transporter-expressing cell systems, LLC-PK1 cells and Xenopus oocytes. Both OAT1 and OAT3 preferred BH(2) and sepiapterin as their substrate roughly 5- to 10-fold more than BH(4). Administration of probenecid acutely reduced the urinary exclusion of endogenous BP accompanied by a rise in blood BP in vivo. These results indicated that OAT1 and OAT3 played crucial roles: (1) in determining baseline levels of blood BP by excluding endogenous BP through the urine, (2) in the rapid distribution to organs of exogenous BH(4) and the exclusion to urine of a BH(4) excess, particularly when BH(4) was administered, and (3) in scavenging blood BH(2) by cellular uptake as the gateway to the salvage pathway of BH(4), which reduces BH(2) back to BH(4).