BIOCHEMICAL FUNCTIONS AND CLINICAL IMPORTANCE OF UNCONJUGATED PTERIDINES

Summary . Tetrahydrobiopterine (BH 4 ) and its relatives are classified as unconjugated pteridines or pterins distinguishing them from the folates. BH 4 is not a vitamin for mammals, since they can synthesize it. GTP is the major precursor of atoms in the pterin nucleus. The initial step in this pathway is conversion of GTP to D-erythro-7, 8-dihydroneopterin triphosphate, a reaction catalyzed by the enzyme GTP-cyclohydrolase I (EC 3.5.4.16; GTP-CH). There are many important metabolic functions of BH 4 : it is a crucial cofactor in hydroxylation reactions of phenylalanine, tyrosine and tryptophane. The finding of BH 4 participation in monoaminergic neurotransmitter metabolism regulation contributed to the knowledge of atipic neurological symptoms in some kinds of "phenylketonuric" children. As a cofactor of nitric oxide syntheses BH 4 is a crucial metabolite involved in physiological function of cardiovascular system. The literature data confirm that the BH 4 depletion is crucial in the control of both NO and superoxide generation (H