Effect of phenylalanine and p-chlorophenylalanine on Na+,K+-ATPase activity in the synaptic plasma membrane from the cerebral cortex of rats

Na+,K+-ATPase activity was measured In synaptic plasma membrane from cerebral cortex of Wistar rats subjected to experimental phenylketonuria, i.e., chemical hyperphenylalaninemia induced by subcutaneous administration of 5.2 mu mol phenylalanine / g body weight (twice a day) plus 0.9 mu mol p-chlorophenylalanine / g body weight (once a day). The treatment was performed from the 6(th) to the 14(th) postpartum day and rats were killed 12 h after the last injection. Synaptic plasma membrane from cerebral cortex was prepared by a discontinuous density sucrose gradient for Na+,K+-ATPase activity determination. The results showed that the enzyme activity was decreased by 30% in animals subjected to experimental phenylketonuria when compared to control. The in vitro effects of the drugs on Na+,K+-ATPase activity were also investigated. Phenylalanine and p-chlorophenylalanine inhibited the enzyme activity and this inhibition was reversed by alanine. In addition, competition between phenylalanine and p-chlorophenylalanine for binding to the enzyme was observed, suggesting a common binding site for these substances. Our results suggest that reduction of Na+,K+-ATPase activity may be one of the mechanisms related to the brain dysfunction observed in human PKU.