We have recently found that the neuroblastoma cell line SH-SY5Y expresses a novel form of phosphate activated glutaminase (PAG) which deamidates glutamine to glutamate and ammonia at high rates. Glutamate production is enhanced during the exponential phase of growth, and decreases when cell proliferation stops. Neuroblastoma PAG exists in a soluble and membrane associated form, and both the phosphate and the glutamine kinetics, as well as the effects of ammonia and glutamate are different from those of the known forms of PAG. Neuroblastoma PAG is mitochondrial, and our immunoblotting analyses of isolated mitochondria shows that our C-terminal antibody reacts with a protein of 65 kDa, while our N-terminal antibody primarily labels a protein of 58 kDa and to a minor degree one of 65 kDa. This strongly suggests that neuroblastoma cells mainly contain an active isoform of PAG lacking the C-terminal end, probably the GAC form.