has role 5,6,7,8-tetrahydrobiopterin diagnostic agent sapropterin A tetrahydropterin that is 2-amino-5,6,7,8-tetrahydropteridin-4(3<em>H</em>)-one in which a hydrogen at position 6 is substituted by a 1,2-dihydroxypropyl group (6<i>R</i>,1'<i>R</i>,2'<i>S</i>-enantiomer).