Tolerance of protein and lipid synthesis to mild hyperphenylalaninemia in developing rat brain.

Chronic injection of low levels (1.5-2.5 mg/g body wt./day for 6-8 days) of L-phenylalanine which did not significantly affect body weight or brain to body weight ratios also did not significantly affect myelin accumulation or synthesis in developing rat brain. A single injection of L-phenylalanine (1 mg/g body wt.) significantly reduced in vivo protein synthesis by 10% in cerebral cortex of 7-day-old rats. Although transient reductions in protein synthesis are produced during low level daily injections, cumulative effects are not expressed in significant reductions in brain to body growth ratios. It is concluded that myelination is significantly inhibited in hyperphenylalaninemia only when other parameters of brain growth and development are affected.