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

Abstract

Tetrahydrobiopterin (BH4) is an essential cofactor of nitric oxide synthase (NOS) and aromatic amino acid hydroxylases. BH4 and 7,8-dihydrobiopterin (BH2) are metabolically interchangeable at the expense of NADPH. Exogenously administered BH4 can be metabolized by the body, similar to vitamins. At present, synthetic BH4 is used as an orphan drug for patients with inherited diseases requiring BH4 supplementation. BH4 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 BH4 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. BH4 administration caused a considerable decrease in the BH4% 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 BH4% was further lowered, suggesting that the probenecid-sensitive BP uptake played a crucial role in BH2 scavenging in vivo. This suggested that the overproduced BH2 was taken up by organs by means of the probenecid-sensitive process, and was then scavenged by counter-conversion to BH4 via the BH4 salvage pathway. Taken together, BH4 administration was effective at raising BP levels in organs over the course of hours but with extremely low efficiency. Since a high BH2 relative to BH4 causes NOS dysfunction, the lowering of the BH4% must be avoided in practice, otherwise the desired effect of the supplementation in ameliorating NOS dysfunction would be spoiled.

Sepiapterin Enhances Tumor Radio- and Chemosensitivities by Promoting Vascular Normalization

“…This investigation used SP instead of BH4 because SP is more permeable to cells (Ohashi et al, 2016). SP has not been used as extensively in the clinic as BH4, with only one reported study on its use in the treatment of atypical phenylketonuria due to BH4 deficiency (Curtius et al, 1979).…”

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

“…Furthermore, the 80% suppression by PBC strongly suggested that the relevant transporters involved in the BH 4 uptake were mostly the organic anion transporters OAT1 and OAT3 which were reported to participate in the removal of various water-soluble compounds in plasma [ 30 – 32 ]. Although the involvement of other transporters in BH 4 removal was not ruled out, the near exclusive participation of the PBC-sensitive transporters in the removal was suggested, and it might be a prerequisite for the rapid release of these compounds to the urine at proximal tubules in rats as observed previously [ 21 , 22 , 29 ].…”

“…The experimental procedure using rats was almost the same as described in the previous paper [ 29 ] except that the rats did not receive 6RBH 4 . In brief, rats (SD, 8–10-week-old males) were loaded with PBC (200 mg/kg) under anesthesia.…”

“…Systemic BH 2 scavenging mechanism. In our latest work on the pharmacokinetics of BH 4 administration, PBC-sensitive transporter(s) was/were shown to be the key transporter in removing BH 2 from the blood [ 29 ]. It was also suggested that the PBC-sensitive transporter played a key role in lowering the BH 2 /BH 4 ratio which would otherwise be raised by BH 4 administration.…”

Tetrahydrobiopterin treatment in phenylketonuria: A repurposing approach

“…Next to this, BH 4 is an instable molecule that is easily oxidized to BH 2 , although it appears BH 2 is then intracellularly reconverted into BH 4. Overall, BH 4 supplementation does increase plasma BH 4 concentrations, albeit very inefficiently, but it is unclear to what extent BH 4 then crosses the blood‐brain barrier (BBB). At least in part, this question is prompted by the experience of BH 4 treatment in patients with BH 4 deficiency, resulting in a disturbed function of PAH, as well as of tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH).…”