Expression of the Mitochondrial Genome in HeLa Cells XVII. Heterogeneity of Isolated HeLa Cell Mitochondria As Assayed For Their Enzymatic and In Vivo Biosynthetic Activities

Abstract

The distributions in dextran-sucrose or sucrose gradients of biosynthetic and enzymatic activities associated with HeLa cell mitochondria have been analyzed. Mitochondria assayed for their in vivo DNA, RNA, or protein synthetic activity exhibit in such gradients closely corresponding sedimentation profiles, which are all displaced to the heavy side with respect to that of the bulk of mitochondria tested by cytochrome c oxidase activity. The sedimentation profile of mitochondria assayed by DNA content follows fairly precisely that of mitochondria assayed by macromolecular synthetic activities. The distribution in a dextran-sucrose gradient of mitochondria assayed by malate dehydrogenase activity is intermediate between those of mitochondria identified by cytochrome oxidase or in vivo macromolecular biosynthetic activities. An electron microscope examination of the structures fractionated in a dextran-sucrose gradient has given results, which, when correlated with the biochemical data, indicate that HeLa cell mitochondria are active in macromolecular syntheses in proportion to their size, while cytochrome oxidase activity is concentrated in smaller mitochondria. The possibility that an in vivo heterogeneity of HeLa cell mitochondria in DNA content and enzyme activities is responsible for the observed differences in mitochondrial distributions in the dextran-sucrose or sucrose gradients is discussed.