Transamination of Amino Acids in Homogenates of Rat Brain

Abstract

Abstract— The aminotransferase activity of homogenates of brains from adult and neonatal rats has been investigated. Aminotransferase activity was demonstrated wtih 15 of 22 amino acids incubated with seven keto acids. The basic amino acids exhibited little or no activity. The greatest activity was obtained when glutamate or aspartate was incubated with α‐ketoglutarate or oxaloacetate. Significant activity was also observed when the neutral aliphatic and aromatic amino acids were incubated with these two keto acids. Activity with pyruvate was obtained principally upon incubation with glutamate and alanine. Most of the other amino acids that underwent transamination with α‐ketoglutarate also did so with pyruvate, although at a lower rate. When phenylpyruvate was added to the medium, glutamate, phenylalanine and tyrosine transaminated most actively. Incubations with 11 amino acids and glyoxylic acid demonstrated aminotransferase activity, with glutamate and ornithine being the most active substrates. α‐Ketoisocaproate and α‐ketoisovalerate accepted amino groups primarily from the branched‐chain amino acids. Except for glutamate, activity with other amino acids was low or not detectable. A comparison of aminotransferase activity in the newborn brain with that in the adult brain showed that the greatest change in activity occurred for glutamate with pyruvate or for alanine with α‐ketoglutarate, these activities increasing about 10‐fold from birth to adulthood; during this time activities with most other amino acids increased two‐ to threefold. Amino transfers from the branched‐chain amino acids showed no increase with maturation, and some reactions, such as that with methionine and a number of keto acids, decreased from birth to adulthood. Our results correspond in general to previous studies of aminotransferase activity in brain and in liver. However, our study also indicates a possible second aminotransferase acting on the branched‐chain amino acids, the presence of aminotransferase activity for methionine and asparagine, and relatively high aminotransferase activity for glutamine or ornithine when incubated with glyoxylic acid rather than other keto acids. Moreover, phenylpyruvate and glyoxylate are active in amino transfers and may serve as substrates for a number of aminotransferases.

Brain Alanine Formation as an Ammonia-Scavenging Pathway during Hyperammonemia: Effects of Glutamine Synthetase Inhibition in Rats and Astrocyte—Neuron Co-Cultures

“…For both aspartate and alanine intracellular 15 N-labeling is 40% to 50% in the co-cultures demonstrating the relevance of both transamination reactions. Even though this is in contrast to findings of relatively low activity of ALAT compared with AAT in homogenates of rat brain, 34 a cell culture study found that ALAT activity is cell-type specific with a three-fold higher enzyme activity in astrocytes compared with neurons. 35 Since aspartate is not released from healthy astrocytes owing to glutamate transporters generating a large inward gradient of glutamate as well as aspartate, 36 this amino acid cannot be considered an alternative ammonia scavenger.…”

In vivo 13C saturation transfer effect of the lactate dehydrogenase reaction

“…No significant signal change in alanine C2 signal at 50.1 ppm was observed in either the bicuculline-treated or the glioma rats upon saturation of pyruvate C2 at 207.9 ppm during [2-13 C]glucose infusion (data not shown). The difficulty of detecting the saturation transfer effect catalyzed by alanine aminotransferase (EC 2.6.1.2) should be due to the intrinsic low activity of alanine aminotransferase in brain tissue in both alanine 3 pyruvate and pyruvate 3 alanine directions (39).…”

Inverse polarization transfer for detecting in vivo 13C magnetization transfer effect of specific enzyme reactions in 1H spectra

“…Cancellation of the elevated Ala signal at 1.47 ppm in the difference spectrum was incomplete due to partial saturation of the Ala C3 signal at 16.8 ppm during the control experiment with the irradiation frequency placed at 12.4 ppm. Note that the rate of alanine aminotransferase reaction is too low in brain tissue to generate any detectable magnetization transfer effect in vivo as explained earlier [2,28]. Fig.…”

“…The relative standard deviations of the reported percent change are in agreement with the predicted values using eq. [28] given by Alger and Shulman [29].…”