Phenylalanine, an essential amino acid, is the precursor to amino acid tyrosine, which helps regulate dopamine, noradrenaline and adrenaline. Found naturally in breast milk, phenylalanine is said to have analgesic and antidepressant qualities. Phenylalanine hydroxylase (PAH) is the enzyme that catalyzes phenylalanine to tyrosine. Mutations in PAH can cause phenylketonuria (PKU), the inability to metabolize phenylalanine and a genetic disorder that can lead to intellectual disability, seizures, motor disorder and skin rashes when not monitored. Because of this association of genetic variations to cognitive ability, researchers at the National Center of Neurology and Psychiatry in Japan set out to test the effect of variations, or single nucleotide polymorphisms, of PAH on memory performance in humans.

For the experiment, researchers Toshiya Teraishi et al. used 599 healthy participants. All participants underwent interviews to make sure they had no history with psychological disorders. They were then given the Wechsler Memory Scale-Revised to evaluate memory performance in five parts: verbal memory, visual memory, general memory, attention and delayed recall. Blood samples were also taken to determine the genomic DNA of the participants and tag six different single nucleotide polymorphisms (SNP) of PAH.

The effect of variations of PAH on memory

After analyzing the test scores and associated SNPs of phenylalanine hydroxylase of the participants, researchers Teraishi et al. found a significant association of the verbal memory set of the score with five SNPs. This suggests that common variations in PAH, specifically variations with SNP marker rs2037639 and haplotype markers rs2037639 and rs10860936, can have an effect on verbal memory performance.

Because SNP rs2037639 was found in previous studies to be associated with schizophrenia in sample of Bulgarian men, the researchers believe that the particular SNP variation increases an individual’s susceptibility to the mental disorder by affecting memory performance.

They report that verbal memory is one of the most altered neurocognitive function for schizophrenics.