RESUMO
Na(+), K(+)-ATPase activity was determined in erythrocyte membranes from 12 phenylketonuric patients of both sexes, aged 8.8 +/- 5.0 y, with plasma phenylalanine levels of 0.64 +/- 0.31 mM. The in vitro effects of phenylalanine and alanine on the enzyme activity in erythrocyte membranes from healthy individuals were also investigated. We observed that Na(+), K(+)-ATPase activity was decreased by 31% in erythrocytes from phenylketonuric patients compared with normal age-matched individuals (p < 0.01). We also observed a significant negative correlation between erythrocyte Na(+), K(+)-ATPase activity and plasma phenylalanine levels (r = -0.65; p < 0.05). All PKU patients with plasma phenylalanine levels higher than 0.3 mM had erythrocyte Na(+), K(+)-ATPase activity below the normal range. Phenylalanine inhibited in vitro erythrocyte Na(+), K(+)-ATPase activity by 22 to 34%, whereas alanine had no effect on this activity. However, when combined with phenylalanine, alanine prevented Na(+) K(+)-ATPase inhibition. Considering that reduction of Na(+), K(+)-ATPase activity occurs in various neurodegenerative disorders leading to neuronal loss, our previous observations showing a significant reduction of Na(+), K(+)-ATPase activity in brain cortex of rats subjected to experimental phenylketonuria and the present results, it is proposed that determination of Na(+), K(+)-ATPase activity in erythrocytes may be a useful peripheral marker for the neurotoxic effect of phenylalanine in phenylketonuria.
Assuntos
Membrana Eritrocítica/enzimologia , Fenilcetonúrias/sangue , Fenilcetonúrias/enzimologia , ATPase Trocadora de Sódio-Potássio/sangue , Adolescente , Estudos de Casos e Controles , Criança , Pré-Escolar , Feminino , Humanos , MasculinoRESUMO
The in vitro effects of phenylalanine or alanine alone or combined on Na+, K+-ATPase activity in membranes from human platelets were investigated. The enzyme activity was assayed in membranes prepared from platelet-rich plasma of healthy donors. Phenylalanine or alanine were added to the assay to final concentrations of 0.3 to 1.2 mM, similar to those found in plasma of phenylketonuric patients. Phenylalanine inhibited Na+, K+-ATPase activity by 20-50% [F(4,25)=11.47 ; p<0.001]. Alanine had no effect on Na+, K+-ATPase activity but when combined with phenylalanine prevented the enzyme inhibition. These results, allied to others previously reported on brain Na+, K+-ATPase activity, may reflect a general inhibitory effect of phenylalanine on this important enzyme activity. Therefore, it is possible that measurement of Na+, K+-ATPase activity in platelets from PKU patients may be a useful peripheral marker for the neurotoxic effects of phenylalanine.