Phenylalanine supplementation improves the phenylalanine profile in tyrosinaemia

Tyrosinaemia types I and II are caused by enzyme deficiencies in the tyrosi ne catabolism pathway. Successful treatment is possible with the novel enzy me inhibitor NTBC in tyrosinaemia type I and with dietary tyrosine and phen ylalanine restriction in both conditions. This is achieved with a low natur al protein intake and a supplementary amino acid formula that is phenylalan ine- and tyrosine-free. Patients on this regimen had been noted, periodical ly, to have very low plasma phenylalanine concentrations (< 20 mu mol/L). T he tyrosine and phenylalanine profiles in six patients were measured. Five of the six patients had very low concentrations of phenylalanine during the later half of the day. The response to phenylalanine supplementation was a ssessed and supplementing the diet with phenylalanine 30-40 mg/kg per day r esulted in normal concentrations throughout the day. Possible complications of hypophenylalaninaemia and potential preventive treatment strategies are discussed. Further studies are needed to investigate the longer-term clini cal and biochemical consequences of phenylalanine supplementation.