NEURONAL METABOLISM OF BRANCHED-CHAIN AMINO-ACIDS - FLUX THROUGH THE AMINOTRANSFERASE PATHWAY IN SYNAPTOSOMES

The metabolism of branched-chain amino acids (BCAAs) was studied in co rtical synaptosomes, is With [N-15]leucine (1 mM) as precursor, the cu mulative appearance of N-15 in [N-15]glutamate and [N-15]aspartate was 0.2 nmol/min/mg of protein without supplemental alpha-ketoglutarate a nd 0.3 nmol/min/mg of protein in the presence of alpha-ketoglutarate ( 0.5 mM). The BCAA aminotransferase reaction also proceeded in the ''re verse'' direction [alpha-ketoisocaproate (KIC) + glutamate --> leucine + alpha-ketoglutarate]. This was documented by incubating synaptosome s with [N-15]glutamate and measuring the formation of [N-15]leucine. W ithout KIC in the medium, the rate of [N-15]leucine production was 0.1 3 nmol/min/mg of protein. In the presence of 25 mu M KIC the rate was 0.79 nmol/min/mg of protein and even greater (1.0 nmol/min/mg of prote in) in the presence of 500 mu m KIC. The reamination of KIC was two- t o threefold faster with [2-N-15]glutamine as precursor compared with [ N-15]glutamate. The ketoacid of valine, alpha-ketoisovalerate (KIV), w as reaminated to [N-15]valine at a rate comparable to that observed wi th respect to KIC. The BCAA transaminase mediated not only the bidirec tional transfer of amino groups between leucine or valine and glutamat e, but also the direct transfer of nitrogen between leucine and valine . This was ascertained in studies in which the incubation medium was s upplemented with either [N-15]leucine and KIV or [N-15]valine and KIC (amino acids at 1 mM and ketoacids at 25 or 500 mu M). The rate was fa ster in the direction of leucine formation at both the lower (6.1-fold ) and higher (1.7-fold) KIC concentration. It is suggested that in syn aptosomes the BCAA transaminase (a) functions predominantly in the dir ection of leucine formation and (b) maintains a constant ratio of BCAA s and ketoacids to one other.