Tetrahydrobiopterin Supplementation: Elevation of Tissue Biopterin Levels Accompanied by a Relative Increase in Dihydrobiopterin in the Blood and the Role of Probenecid-Sensitive Uptake in Scavenging Dihydrobiopterin in the Liver and Kidney of Rats

Tetrahydrobiopterin (BH(4)) is an essential cofactor of nitric oxide synthase (NOS) and aromatic amino acid hydroxylases. BH(4) and 7,8-dihydrobiopterin (BH(2)) are metabolically interchangeable at the expense of NADPH. Exogenously administered BH(4) can be metabolized by the body, similar to vitamins. At present, synthetic BH(4) is used as an orphan drug for patients with inherited diseases requiring BH(4) supplementation. BH(4) supplementation has also drawn attention as a means of treating certain cardiovascular symptoms, however, its application in human patients remains limited. Here, we tracked biopterin (BP) distribution in blood, bile, urine, liver, kidney and brain after BH(4) administration (5 mg/kg rat, i.v.) with or without prior treatment with probenecid, a potent inhibitor of uptake transporters particularly including organic anion transporter families such as OTA1 and OAT3. The rapid excretion of BP in urine was driven by elevated blood concentrations and its elimination reached about 90% within 120 min. In the very early period, BP was taken up by the liver and kidney and gradually released back to the blood. BH(4) administration caused a considerable decrease in the BH(4)% in blood BP as an inevitable compensatory process. Probenecid treatment slowed down the decrease in blood BP and simultaneously inhibited its initial rapid excretion in the kidney. At the same time, the BH(4)% was further lowered, suggesting that the probenecid-sensitive BP uptake played a crucial role in BH(2) scavenging in vivo. This suggested that the overproduced BH(2) was taken up by organs by means of the probenecid-sensitive process, and was then scavenged by counter-conversion to BH(4) via the BH(4) salvage pathway. Taken together, BH(4) administration was effective at raising BP levels in organs over the course of hours but with extremely low efficiency. Since a high BH(2) relative to BH(4) causes NOS dysfunction, the lowering of the BH(4)% must be avoided in practice, otherwise the desired effect of the supplementation in ameliorating NOS dysfunction would be spoiled.