Bovine kidney mitochondria were separated into matrix and membrane fractions by treatment with digitonin and Lubrol PX. While malate dehydrogenase was found essentially in the matrix fraction, both the pyruvate and the α-oxoglutarate dehydrogenase multienzyme complexes remained bound to the inner membrane fraction and became solubilized only after repeated treatments with detergents. Thus both multienzyme complexes must be associated with the inner membrane rather than located within the matrix space.
