Evidence for Compartmentation of Synaptosomal Phosphate‐Activated Glutaminase

: Phosphate‐activated glutaminase (EC 3.5.1.2) in synaptosomal preparations is inhibited 40–60% by the sulphydryl group reagent N‐ethylmaleimide (NEM), forming the basis for distinction between NEM‐sensitive and NEM‐insensitive glutaminases. The NEM effect cannot be explained by differential effects on distinct glutaminases because other glutaminases have not been detected, and the synaptosomal glutaminase activity can be fully accounted for by the activity of phosphate‐activated glutaminase. By fractionation of mitochondria isolated from synaptosomal preparations, which are preincubated with and without NEM, both NEM‐sensitive and NEM‐insensitive glutaminases are found to be localized to the inner mitochondrial membrane. Variations in pH (7.0–7.6) and the phosphate concentration (5–10 mM) affect chiefly NEM‐sensitive glutaminase, demonstrating that this glutaminase may be subject to regulation by compounds in the cytosol having restricted permeability to the inner mitochondrial membrane. Since p‐hydroxymercuribenzoate, which is known to be impermeable to the inner mitochondrial membrane, inhibits glutaminase similarly to NEM, phosphate‐activated glutaminase is assumed to be compartmentalized within the inner mitochondrial membrane. Thus, NEM‐sensitive glutaminase is localized to the outer face and NEM‐insensitive glutaminase to the inner region of this membrane and probably also to the matrix region.