Transport of α-Ketoisocaproate in Rat Cerebral Cortical Neurons

Transport of α-ketoisocaproic acid (KIC), the product of leucine transamination, was studied in the cerebral cortex cells isolated from the adult rat brain. The process of [14C]KIC accumulation was followed in the presence of aminooxyacetate, an inhibitor of transaminases. Accumulation of KIC was not affected by Na+ replacement, its initial velocity was observed to be higher upon lowering of external pH. Addition of KIC promoted acidification of cytoplasmic pH, monitored with 2′7′-bis(carboxyethyl)-5(6)-carboxyfluorescein. The detected inhibition of KIC accumulation by α-cyano-4(OH)cinnamate pointed to an involvement of one of monocarboxylate transporters (MCT), although 4,4′-diisothiocyano-2,2′-stilbenedisulphonate was without effect. Accumulation of KIC was inhibited by lactate; the effect of pyruvate was detected to be much weaker. Other branched-chain α-ketoacids (ketoisovalerate, keto-methylvalerate), as well as β-hydroxybutyrate and valproate decreased the transport of KIC by 30, 60, and 80%, respectively. The observed characteristics of KIC accumulation in the cortical neurons indicate an involvement of one of the MCT transporters. A crucial role of SH group(s) in the process of KIC accumulation, excluding MCT2, indicates the MCT1, although an involvement of another isoform of MCT in the process of KIC transport in neurons from cerebral cortex of adult brain has not been definitely excluded.