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

Tetrahydrobiopterin (BH) is a common coenzyme of phenylalanine-, tyrosine-, and tryptophan hydroxylases, alkylglycerol monooxygenase, and NO synthases (NOS). Synthetic BH is used medicinally for BH-responsive phenylketonuria and inherited BH deficiency. BH 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, the oxidized form of BH. An increase in the BH/BH ratio leads to NOS dysfunction. Previous studies revealed that BH supplementation caused a rapid urinary loss of BH accompanied by an increase in the blood BH/BH 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 and/or BH) 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 and sepiapterin as their substrate roughly 5- to 10-fold more than BH. 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 and the exclusion to urine of a BH excess, particularly when BH was administered, and (3) in scavenging blood BH by cellular uptake as the gateway to the salvage pathway of BH, which reduces BH back to BH.