HYPERPHENYLALANINEMIA AND PTERIN METABOLISM IN SERUM AND ERYTHROCYTES

The relationship between blood phenylalanine concentrations and serum and erythrocyte biopterin and neopterin concentrations was investigate d in 20 phenylketonuric patients with different dietary compliance. At serum phenylalanine concentrations ranging from 43 to 1004 mumol/l, a good correlation was found with serum biopterin (r = 0.76, P < 0.001) and with red blood cell biopterin (r = 0.62, P < 0.001). A similar co rrelation was found between serum neopterin and phenylalanine (r = 0.6 0, P < 0.001). The correlation between red blood cell neopterin and se rum phenylalanine was less evident, however (r = 0.47, P < 0.005). Aft er oral loading with phenylalanine (100 mg/kg body weight), serum and red blood cell biopterin concentrations increased in patients with cla ssical phenylketonuria as well as in one patient with dihydropteridine reductase deficiency in response to the induced acute hyperphenylalan inemia. One patient suffering from 6-pyruvoyl tetrahydropterin synthas e deficiency was loaded orally with tetrahydrobiopterin (20 mg/kg body weight). The kinetics of administered cofactor confirmed its rapid ab sorption, with early increase of serum concentrations followed by its transport into the red blood cells. The half-life of biopterin was app roximately 7 h in serum and 15 h in red blood cells. Because both valu es are less than the half-life of phenylalanine (20-30 h) in serum, bi opterin measurement offers no advantage in monitoring dietary control in hyperphenylalaninemic patients.