Factors affecting the access of mitochondrial kynurenine aminotransferase to its substrates.

The kynurenine aminotransferase activity of isolated rat kidney mitochondria is enhanced several-fold by membrane disruption or by the addition of 1 mM CaCl2. The addition of CaCl2 can thus be used conveniently to obtain full expression of the mitochondrial enzyme activity. CaCl2 apparently stimulates the activity by facilitating the penetration of the mitochondrial inner membrane by the substrate, alpha-ketoglutarate. Consistent with this view, two inhibitors of dicarboxylate translocation, benzylmalonate and quinolinate, inhibited the activity of the isolated mitochondria but not that of the disrupted mitochondria. Both inhibitions were prevented at higher Ca++ concentrations, suggesting that thes compounds inhibit by complexing Ca++ needed for the translocation of alpha-ketoglutarate. This action of quinolinate may explain its ability to interfere in vivo with the mitochondrial translocation of dicarboxylates as reported by Spydevold et al.