High levels of
phenylalanine (Phe) are the biochemical hallmark of
phenylketonuria (PKU), a neurometabolic disorder clinically characterized by severe
mental retardation and other brain abnormalities, including cortical
atrophy and
microcephaly. Considering that the pathomechanisms leading to brain damage and particularly the marked
cognitive impairment in this disease are poorly understood, in the present study we investigated the in vitro effect of Phe, at similar concentrations as to those found in brain of PKU patients, on important parameters of oxidative stress in the hippocampus and cerebral cortex of developing rats. We found that Phe induced in vitro lipid peroxidation (increase of TBA-RS values) and
protein oxidative damage (sulfhydryl oxidation) in both cerebral structures. Furthermore, these effects were probably mediated by
reactive oxygen species, since the
lipid oxidative damage was totally prevented by the
free radical scavengers alpha-tocopherol and
melatonin, but not by
L-NAME, a potent inhibitor of
nitric oxide synthase. Accordingly, Phe did not induce
nitric oxide synthesis, but significantly decreased the levels of
reduced glutathione (GSH), the major brain
antioxidant defense, in hippocampus and cerebral cortex supernatants. Phe also reduced the
thiol groups of a commercial GSH
solution in a cell-free medium. We also found that the major metabolites of Phe catabolism,
phenylpyruvate, phenyllactate and
phenylacetate also increased TBA-RS levels in cerebral cortex, but to a lesser degree. The data indicate that Phe elicits oxidative stress in the hippocampus, a structure mainly involved with learning/memory, and also in the cerebral cortex, which is severely damaged in PKU patients. It is therefore presumed that this pathomechanism may be involved at least in part in the severe cognitive deficit and in the characteristic cortical
atrophy associated with dysmyelination and leukodystrophy observed in this disorder.