Distribution patterns of membrane-bound and soluble enzymes in mitochondrial populations.

Mitochondria were isolated from normal rat liver, kidney, and heart, and from mouse liver and ascites tumor cells, and were dispersed through linear sucrose density gradients in a zonal centrifuge. Distributions of the enzyme activity with respect to mean particle size were constructed. The medians of the distributions for the activities of several enzymes associated with the membranes were significantly different from the medians of the distributions of the soluble mitochondrial enzymes when the mean particle diameters of the fractions were used as the measure of particulate size. On the other hand, when the activity of an outer-membrane enzyme was determined as a function of the area of the mitochondria and the activities of the soluble enzymes were expressed as a function of the volume, the calculated particle diameters corresponding to the respective midpoints of these distributions were in better agreement. This congruence suggests that mitochondria are more nearly homogeneous with respect to the enzyme activities examined than previously proposed. Furthermore, since the distribution of the inner membrane activities was similar to those of the outer membrane enzymes, the area of the inner membrane may be proportional, not to the volume of the mitochondria, but to the area of the outer membrane.