Tyrosine transport is regulated differently in patients with schizophrenia

Previous PET studies of tyrosine transport have suggested that the transpor t of tyrosine from blood to brain compartment is not dependent on its plasm a concentration in patients with schizophrenia. In order to examine this relationship, the transport constant (K-1) of tyro sine was determined in five patients with schizophrenia and five normals. L -[1-C-11]Tyrosine was injected i.v. and arterial blood samples were taken d uring PET scanning. The tyrosine transport was assessed during baseline con ditions and after oral administration of L-tyrosine at a dose (175 mg/kg) t hat significantly elevated the plasma levels. K-1 was determined from trace r kinetic modelling. The transport rate dropped in the normals after tyrosine loading, which is consistent with the prevailing notion that the brain transport system for n eutral amino acids works close to saturation, whereas it was virtually unch anged in the schizophrenics. The results demonstrated that tyrosine transpo rt was not saturated in the patients with schizophrenia and thus could lead to elevated brain concentrations of tyrosine. (C) 1999 Elsevier Science B. V. All rights reserved.