Kinetics and localization of brain phosphate activated glutaminase

The cellular concentration of phosphate, the main activator of phosphate activated glutaminase (PAG) is rather constant in brain and kidney. The enzyme activity, however, is modulated by a variety of compounds affecting the binding of phosphate, such as glutamate, calcium, certain long chain fatty acids, fatty acyl CoA derivatives, members of the tricarboxylic acid cycle and protons (Kvamme et al. [2000] Neurochem. Res. 25:1407-1419). Therefore, the kinetic and allosteric properties of the enzyme are essential for regulating the enzyme activity in situ, especially because the enzymically active pool of PAG is assumed to have an external localization in the inner mitochondrial membrane, being exposed to cytosolic variation in the content of effectors. This has largely been overlooked. A hypothetical model for the allosteric interactions based on the sequential induced fit allosteric model by Koshland et al. ([1966] Biochemistry 5:365-385) is presented. Furthermore, it has been generally accepted that there exist only two isoforms of PAG, the kidney PAG that is similar to brain PAG, and the liver PAG. Therefore, the immunoreactivity of brain cells against kidney PAG antibodies has been considered a measure of PAG protein. Gomez-Fabre et al. ([2000] Biochem. J. 345:365-375) recently found, however, that a PAG mRNA from human breast cancer ZR75 cells is present in human brain and liver, but not in the kidney. We observed only traces of PAG immunoreactivity in cultured astrocytes and cultured neuroblastoma cells, regardless whether antibodies against the C- and N-termini of kidney PAG or antibodies against liver PAG were used, but considerable enzyme activity, demonstrating hitherto unknown isoforms of PAG (Torgner et al. [2001] FEBS Lett. 268(Suppl 1):PS2-031).