Reduced 6,6,8-trimethylpterins. Preparation, properties and enzymic reactivities with dihydropteridine reductase, phenylalanine hydroxylase and tyrosine hydroxylase

The substrates of dihydropteridine reductase (EC 1.6.99.7), quinonoid 7,8-dihydro(6 H)pterins, are unstable and decompose in various ways. In attempting to prepare a more stable substrate, 6,6,8-trimethyl-5,6,7,8-tetrahydro(3 H)pterin was synthesised and the quinonoid 6,6,8-trimethyl-7,8-dihydro(6 H)pterin derived from it is extremely stable with a half-life in 0.1 M Tris/HCl (pH 7.6, 25 degrees C) of 33 h. Quinonoid 6,6,8-trimethyl-7,8-dihydro(6 H)pterin is not a substrate for dihydropteridine reductase but it is reduced non-enzymically by NADH at a significant rate and it is a weak inhibitor of the enzyme: I50 200 microM, pH 7.6, 25 degrees C when using quinonoid 6-methyl-7,8-dihydro(6 H)pterin as substrate. 6,6,8-Trimethyl-5,6,7,8-tetrahydropterin is a cofactor for phenylalanine hydroxylase (EC 1.14.16.1) with an apparent Km of 0.33 mM, but no cofactor activity could be detected with tyrosine hydroxylase (EC 1.14.16.2). Its phenylalanine hydroxylase activity, together with the enhanced stability of quinonoid 6,6,8-trimethyl-7,8-dihydro(6 H)pterin, suggest that it may have potential for the treatment of variant forms of phenylketonuria.