Abstract: | 6α,β-Tetrahydro-L -[3′-2H1]biopterin ([3′-2H1]- 1 ) was administered orally to two primapterinuric patients in order to investigate the biosynthetic pathway of 7-substituted pterins in humans. L -Primapterin ( 2 ) and L -biopterin were isolated from urine after loading and measured by GC/MS. L -Biopterin and L -primapterin were labelled with 2H to an equal extent. From this result, one can conclude that L -primapterin is formed from tetrahydro-L -biopterin, very probably via an intramolecular rearrangement. |