Determination of the orientation of membrane vesicles derived from mitochondria.

Membrane vesicles of physiological as well as inverted orientation can be isolated from mitochondria. The presence of these vesicles in a membrane can be determined and quantitated by determining the differences between the two vesicle types in terms of rates of NADH oxidation, rates of oxidation of tricarboxylate cycle intermediates, rates of ATP hydrolysis and sensitivity to inhibitors, stimulation of respiration by exogenous cytochrome c, inhibition of respiration by polycationic proteins, and visualization of the ATPase by electron microscopy. Procedures to isolate the two membrane types and characteristics of homogeneously oriented preparations are described. Differences in data obtained with homogeneous vesicle preparations and with vesicles of mixed orientation are illustrated. Nonhomogeneously oriented preparations can be enriched in the desired vesicular type by the use of immunoprecipitation, affinity chromatography, and differential centrifugation. The concept of a hybrid vesicle containing oppositely oriented regions is not supported by experimental data.