Synaptosomal transport of branched chain amino acids in young, adult and aged rat brain cortex.

Uptake of branched chain amino acids (BCAA, leucine and isoleucine) was studied in synaptosomes prepared from the cerebral cortex of rats of 1, 3 and 24 months of age. In addition to the conventional low affinity sodium independent transport system, a high affinity sodium dependent stereospecific transport system for the transport of BCAA was identified in synaptosomes prepared from the cerebral cortex of the above three age groups. There was an overall decrease in Km and Vmax of both high and low affinity transport systems for leucine and isoleucine in the cortical synaptosomes of 24-month-old rats when compared with younger age groups. This study indicates that the non-neurotransmitter essential amino acids are transported by high and low affinity transport systems and these systems undergo age-dependent alterations. These changes might be due to the altered synthesis of these transporter proteins and/or synthesis of transporters with altered conformation and/or changes in the physical properties (fluidity) of the membrane. The decrease in the transport of BCAA is on a par with the decrease in the overall metabolism of BCAA in brain. As food consumption decreases in the older age groups of animals, the availability of essential amino acids to the tissues might also be lowered. Under such conditions, it is suggested that the observed increase in the affinity (decreased Km) of the carrier might be helpful in the supply of essential amino acids.