Biosynthesis of tetrahydrobiopterin: possible involvement of tetrahydropterin intermediates.

The biosynthetic pathway of tetrahydrobiopterin (BH(4)) from dihydroneopterin triphosphate (NH(2)P(3)) was studied in fresh as well as heat-treated human liver extracts. The question of NAD(P)H dependency for the formation of sepiapterin was examined. NH(2)P(3) was converted by fresh extracts to sepiapterin in low quantities (2% conversion) in the absence of exogenously added NADPH as well as under conditions that ensured the destruction of endogenous, free NAD(P)H. The addition of NADPH to the fresh liver extracts stimulated the synthesis of BH(4) to a much higher yield (17% conversion), and the amount of sepiapterin formed was reduced to barely detectable levels. In contrast, the heat-treated extract (enzyme A2 fraction) formed sepiapterin (1.3% conversion) only in the presence and not in the absence of NADPH. These results indicate that sepiapterin may not be an intermediate on the pathway leading to BH(4) biosynthesis under normal in vivo conditions. Rather, sepiapterin may result from the breakdown of an as yet unidentified intermediate that is actually on the pathway. It is speculated that NH(2)P(3) may be converted to a diketo-tetrahydropterin intermediate (or an equivalent tautomeric structure) by a mechanism involving an intramolecular oxidoreduction reaction. A diketo-tetrahydropterin intermediate could be converted to 5,6-dihydrosepiapterin, which also has a tetrahydropterin ring system and can be converted directly to BH(4) by sepiapterin reductase. This proposed pathway can explain ho the tetrahydropterin ring system can be formed without sepiapterin, dihydrobiopterin, or dihydrofolate reductase being involved in BH(4) biosynthesis in vivo .