The ability of 2-amino-4-hydroxy-7-[dihydroxylpropyl-(L-erythro)-5,6,7,8-tetrahydropterin] ("7-tetrahydrobiopterin" or 7-BH4) to substitute for the natural cofactor tetrahydrobiopterin (BH4) has been studied in vitro in the reactions of the three mammalian aromatic amino acid hydroxylases. With rat liver phenylalanine hydroxylase, the apparent K(m) for 7-BH4 is 160 muM, a value that is almost-equal-to 60-fold greater than that for the natural cofactor. In contrast, the hydroxylase reaction is severely inhibited by as little as 1 muM 7-BH4 when assayed in the presence of physiological concentrations of BH4. This inhibition can be overcome either by an increase in the concentration of BH4 or a decrease in the concentration of phenylalanine. With both rat brain tryptophan hydroxylase and rat pheochromocytoma tyrosine hydroxylase, the K(m) value for 7-BH4 is about one order of magnitude greater than the K(m) for BH4. Accordingly, 7-BH4 is a poor competitive inhibitor of both tryptophan and tyrosine hydroxylase. Thus, our results suggest that the observed hyperphenylalaninemia in patients who excrete 7-BH4 in their urine may arise directly from the inhibition of phenylalanine hydroxylase by low levels of this pterin. On the other hand, it is less likely that low levels of 7-BH4 would affect the activity of tyrosine or tryptophan hydroxylase in vivo.
