Cellular accumulation of tetrahydrobiopterin following its administration is mediated by two different processes; direct uptake and indirect uptake mediated by a methotrexate-sensitive process.

In our previous study on tetrahydrobiopterin (BH4) accumulation in organs of mice administered with 6RBH4, it was demonstrated that the intestinal mucosa was able to take up BH4 directly but that the liver could accomplish this only indirectly via a pathway involving the dihydrofolate reductase reaction. This observation was largely based on the fact that BH4 deposition in the liver was completely inhibited by prior treatment with methotrexate whereas deposition in the intestinal mucosa was only partially inhibited. To investigate the distinctive features of BH4 uptake in these organs, Caco-2 of intestinal epithelial origin and isolated hepatocytes were analyzed for cellular BH4 uptake in vitro.

Accumulated BH4 in mouse liver caused by administration of either 6R- or 6SBH4 consisted solely of the 6R-diastereomer: evidence of oxidation to BH2 and enzymic reduction.

Mice were given (i.p.) L-erythro-(6S)-tetrahydrobiopterin (6SBH4) or 6RBH4 and the increase in liver BH4 in both groups was almost the same.

Tetrahydrobiopterin uptake in supplemental administration: elevation of tissue tetrahydrobiopterin in mice following uptake of the exogenously oxidized product 7,8-dihydrobiopterin and subsequent reduction by an anti-folate-sensitive process.

In order to increase the tissue level of tetrahydrobiopterin (BH4), supplementation with 6R-tetrahydrobiopterin (6RBH4) has been widely employed. In this work, the effectiveness of 6RBH4 was compared with 7,8-dihydrobiopterin (7,8BH2) and sepiapterin by administration to mice. Administration of 6RBH4 was the least effective in elevating tissue BH4 levels in mice while sepiapterin was the best.