FIG. 1.

Stages of mitochondrial enrichment. When measuring mitochondrial ETC enzyme activities from a tissue section the first step is homogenization. There is no mitochondrial enrichment in the homogenate, and most of the protein in the homogenate is not mitochondrial. The first enrichment step involves the removal of nuclei to produce a postnuclear fraction. The supernatant from the postnuclear fraction can be centrifuged to pellet mitochondria and other similar-sized organelles. This stage is called a crude mitochondria fraction. The highest degree of enrichment results after the mitochondria have been separated from other organelles via density gradient centrifugation. With each enrichment step the assayed signal is greater and therefore more sensitive to subtle differences between samples.