POLYRIBOSOME DISAGGREGATION AND CELL‐FREE PROTEIN SYNTHESIS IN PREPARATIONS FROM CEREBRAL CORTEX OF HYPERPHENYLALANINEMIC RATS¹

Abstract

Abstract— Seven‐day‐old rats were injected intraperitoneally with l‐phenylalanine (1 g/kg) and the time course of brain polyribosome disaggregation and changes in brain levels of phenylalanine, tryptophan and tyrosine were determined. Disaggregation of brain polyribosomes preceded the increase in levels of phenylalanine in brain, and followed the same time course as depletion of tryptophan from brain. The effects of several metabolites of phenylalanine (which are formed in phenylketonuria) on protein synthesis in vitro was determined for brain and liver systems. None of the compounds tested was inhibitory at concentrations below 10 mM and in all cases hepatic protein synthesis was more sensitive to inhibition than was the corresponding system from brain. Ribosomal dimers, formed in brain after injection of phenylalanine, were incapable of supporting high levels of protein synthesis in vitro, a finding that suggested that the inhibition of protein synthesis in vitro in cell‐free systems of brain tissue after injection of phenylalanine into young rats was mediated by disaggregation of brain polyribosomes associated with tryptophan deficiency in brain.

Lipid Metabolism in Liver and Brain in Acute Hyperphenylalaninemia in Rats of Different Ages

“…The brain to plasma ratio was approximately one half the liver to plasma ratio at all develop mental ages in both control-and phenylalanine-injected rats. Brain phenyl alanine levels were constant or increasing when plasma levels were de creasing (table II) [39], while liver to plasma ratios were fairly constant during an injection schedule (table II). This indicates that brain phenyl alanine levels are more slowly equilibrated with plasma levels than were liver levels which exhibited a rapid equilibration.…”

“…A very rapid inhibition of protein synthesis after a single injection of phenylalanine (1 mg/g body wt) was found in cerebral cortex from 7-day-old rats but not 28-day-old rats. These effects in 7-day-old rats were correlated with reduced free polyribosome levels and lowered tryptophan content in young rats, while the lack of effect in older rats, when tryptophan levels were also reduced, may be due to a lesser effect on membrane-bound polyribosomes which predominate in older rats [4,39], In 18-day-old rats, a single injection of phenylalanine (2 mg/g body wt) inhibited equally the uptake of 35S-methionine and 14C-leucine into brain protein and amino acid pools [1], The incorporation of label into myelin protein, however, was inhibited somewhat more than incorporation into total protein [1], while a somewhat lesser amino acid incorporation into both brain myelin and nonmyelin proteolipid protein occurred compared to total protein. Adult whole brain chloroform-methanol-soluble protein was also labeled less well (46% of control) compared to chloroform-methanol-insoluble protein (59% of control) in phenylalanine-loaded rats [1], In adult brain, a proteolipid protein fraction (table VIII), which would be primarily derived from myelin, did not show a significant reduction in labeling compared to other fractions.…”

“…A single injection of phenylalanine in 18-day-old rats produces reductions in brain serotonin [G eison, unpublished], while no effects on acetate labeling of lipids is produced [1], These studies suggest that reduction in serotonin levels does not contribute to the observed changes in labeling patterns. Single injections of phenylalanine (1 mg/g) reduced serotonin in 21-day-old but not 7-day-old rats [39], The three in jections (table IV) reduced serotonin levels at all ages studied, although the effects were not as marked on 4-and 7-day-old rats.…”

Effects of Quantity and Quality of Dietary Protein on the Brain Polysome Profile in Aged Rats1

“…Many investigation suggested that the polysome proˆle in tissues represented the changes in the translational phase of protein synthesis, and included the initiation, elongation, and termination in that. [5][6][7]9,26) Therefore, the changes in quantity and quality of dietary protein may have controlled the translational phase of protein synthesis and been one of the factors aŠecting brain protein synthesis in aged rats. In both liver 27,28) and muscle, [28][29][30] the stimulation of protein synthesis caused by amino acids and protein is reported to be mediated by the increase in the initiation of mRNA translation.…”

Ultrastructural evidence against in vivo disaggregation of brain polyribosomes after administration of L-dopa or phenylalanine

“…L-dopa or phenylalanine (Aoki and Siegel, 1970;Maclnnes and Schlesinger, 1971;Siegel et al, 1971;Weiss et aI., 1971Weiss et aI., , 1972). These studies demonstrate that disaggregation of brain polysomes has occurred by the time the end stage in the analytical observation has been reached, but they are not designed to show that the dissociation occurred in the intact animal before removal of brain tissue.…”