No abstract

Phenylketonuria is characterized by a variable degree of mental retardation and other neurological features whose mechanisms are not fully understood. In the present study we investigated the effect of intrahippocampal administration of phenylalanine, isolated or associated with pyruvate or creatine, on rat behavior and on oxidative stress. Sixty-day-old male Wistar rats were randomly divided into 6 groups: saline; phenylalanine; pyruvate; creatine; phenylalanine + pyruvate; phenylalanine + creatine. Phenylalanine was administered bilaterally in the hippocampus one hour before training; pyruvate, at the same doses, was administered in the hippocampus one hour before phenylalanine; creatine was administered intraperitoneally twice a day for 5 days before training; controls received saline solution at same volumes than the other substances. Parameters of exploratory behavior and of emotionality were assessed in both training and test sessions in the open field task. Rats receiving phenylalanine did not habituate to the open field along the sessions, indicating deficit of learning/memory, but parameters of emotionality were normal, not interfering in the habituation process. Pyruvate or creatine administration prevented the lack of habituation caused by phenylalanine. Pyruvate and creatine also prevented alterations provoked by phenylalanine on lipid peroxidation, total content of sulfhydryls, total radical-trapping antioxidant potential and total antioxidant reactivity. The results suggest that the behavioral alterations provoked by intra-hippocampal administration of phenylalanine may be caused, at least in part, by oxidative stress and/or energy deficit. If this also occurs in PKU, it is possible that pyruvate and creatine supplementation to the phenylalanine-restricted diet might be beneficial to phenylketonuric patients.

The main objective of the present study was to characterize the inhibition by phenylalanine and phenylpyruvate of ATP diphosphohydrolase activity in synaptosomes from the brain cortex of rats. This enzyme participates together with a 5'-nucleotidase in adenosine formation from the neurotransmitter, ATP, in the synaptic cleft. The inhibition of ATP diphosphohydrolase was competitive for nucleotide hydrolysis but 5'-nucleotidase was not affected by these metabolites. Furthermore, the two substances inhibited enzyme activity by acting at the same binding site. If the enzyme inhibition observed in vitro also occurs in the brain of PKU patients, it may promote an increase in ATP levels in the synaptic cleft. In this case, the neurotoxicity of ATP could possibly be one of the mechanisms leading to the characteristic brain damage of phenylketonuria.

The nucleotide (ATP-ADP)/nucleoside (adenosine) ratio in the circulation can modulate the processes of vasoconstriction, vasodilatation and platelet aggregation. The main objective of the present study with rat blood serum was to evaluate the possibility of changes in nucleotide hydrolysis by phenylalanine (Phe) and phenylpyruvate (PP), the levels of which could increase in the circulation of individuals with phenylketonuria. Results demonstrated that Phe in the range 1.0-5.0 mM inhibited the ADP hydrolysis by rat serum. The effect of inhibition by Phe on ATP hydrolysis appeared only at a concentration of 5.0 mM. PP had no significant effect upon nucleotide hydrolysis. Kinetic analysis indicated that the inhibition of ADP and ATP hydrolysis by Phe in rat blood serum is uncompetitive. Conversely, Phe and PP did not affect the hydrolysis of p-nitrophenyl-5'-TMP by rat serum.