Biopterin. III. Purification and characterization of enzymes involved in the cerebral synthesis of 7,8-dihydrobiopterin.

Three specific enzymes are involved in the cerebral synthesis of 7,8-dihydrobiopterin from GTP. These were isolated, purified, and characterized. The first enzyme, also catalyzing the rate-limiting step, is GTP-cyclohydrolase A-I or Mg2+-dependent A-II, which hydrolyze the GTP to the specific product 2-amino-6-(5-triphosphoribosyl)-amino-5-or-6-formamido-6-hydroxypyrimidine (FPyd-P3). FPyd-P3 is cyclized by a synthetase to D-erythro-7,8-dihydroneopterintriphosphate (NPTH2-P3). The new enzyme, D-erythro-7,8-dihydroneopterintriphosphate synthetase (enzyme B) is a basic protein of 9177 daltons containing three free SH groups, isoleucyl-seryl- as N- and valyl-glutamyl- as C-terminals. This enzyme of 69 amino acid residues from rat and 68 residues (one less aspartic acid) from guinea pig brain contains no hydroxyproline, methionine, or tryptophan. The enzyme from rat brain will gradually convert its product NPTH2-P3 to BH2, wherease the enzyme from guinea pig brain lacks this property. 2,4-amino-6-hydroxypyrimidine and dFPyd-P3 are effective inhibitors of this enzyme. The synthesis of BH2 from NPTH2-P3, but not from 7,8-dihydroneopterin, is catalyzed by L-erythro-7,8-dihydrobiopterin synthetase (enzyme C), which was purified to electrophoretic purity. This enzyme does not require pyridine nucleotides of Mc2+ for its catalysis.