Regulation of human and rat brain metabolism: Inhibitory action of phenylalanine and phenylpyruvate on glycolysis, protein, lipid, DNA and RNA metabolism

Abstract

In human and rat brain the activity of key glycolytic enzymes and glycolysis were examined during development. The differential inhibitory effect of l-phenylalanine and phenylpyruvate on key glycolytic enzymes, on lactate production and on the incorporation of labeled glucose into lipid, protein, RNA and DNA was investigated in the brain of differentiating human and rat. 1. (1)|In the fetal human brain hexokinase, pyruvate kinase and glycolytic activities were approximately 10% of those observed in the adult. These parameters in the average brain cell gradually rose during differentiation. A similar pattern for these three glycolytic parameters was observed in the differentiating rat brain. 2. (2)|The sensitivity to inhibition by l-phenylalanine and phenylpyruvate was the same order of magnitude for the glycolytic enzymes and lactate production during differentiation. However, since the activities were very low in the fetal and the newborn brain, these parameters may be more vulnerable to inhibition by these compounds that accumulate in the untreated phenylketonuric patients. 3. (3)|The human and fetal brain has a very high cellularity (600–700 millions per g) during gestation and continues to decrease until it reaches a level of about 100 millions of cells per gram wet weight in the adult brain. This emphasizes the importance of expressing biochemical results on a per cell basis during differentiation. 4. (4)|Studies on the mechanism of effect of l-phenylalanine and phenylpyruvate in slices from differentiating human and adult brain were carried out. The kinetic results suggest that these compounds act through a competitive mechanism and the inhibitions were reversible by increasing the substrate level. 5. (5)|The effect of phenylpyruvate on lactate production from glucose, glucose-6-phosphate and fructose-1,6-diphosphate was also explored in the fortified supernatant fluid system from brains of differentiating human and rat. Phenylpyruvate resulted in a progressive inhibition of lactate production from all three substrates. Reciprocal plots, the Dixon plot and the reversibility of the inhibition exerted by phenylpyruvate suggested a competitive type of inhibition. 6. (6)|Since the metabolic inhibitions exerted by phenylalanine and phenylpyruvate on carbohydrate metabolism were reversible, it is possible that the high carbohydrate diet might be able to protect the developing brain from the potential damage of the high levels of these compounds in the phenylketonuric patients. 7. (7)|When increasing concentrations of phenylpyruvate were added to brain slices from differentiating rat, there was a steady, dose-dependent decrease in the incorporation of labeled glucose into lipid, RNA and DNA, which in all cases was more pronounced in slices from the fetal brain than from the adult. It is striking that there was a progressive decrease in the incorporation of label into protein with increasing concentrations of phenylpyruvate in the fetal brain but the adult brain was not affected at any concentration. These data are in agreement with the postulate that phenylpyruvate might interfere with the biosynthesis of strategic macromolecules in the differentiating brain. The results of this work also agree with clinical experience that points to the early stages of brain development as especially vulnerable to toxic effects, including metabolic disturbances. 8. (8)|The enzymatic attacking points of l-phenylalanine and phenylpyruvate involve the inhibition of key enzymes of glycolysis (hexokinase and pyruvate kinase) and of the direct oxidative pathway (6-phosphogluconate dehydrogenase). The inhibition of these enzymes might explain, at least in part, the inhibition of the biosynthesis of key macromolecules in the brain.