Mitochondrial enzyme activity in “senescent” and virus-transformed human fibroblasts

The possible role of changes in mitochondrial content in limiting the in vitro lifespan of human fibroblasts in culture and on the cell line permanence engendered by virus transformation was evaluated. The specific activity of a mitochondrial enzyme, cytochrome oxidase, was determined in a normal human fibroblast strain from shortly after its in vitro cultivation to the cessation of division. Enzyme specific activity was also determined in an SV40-transformed permanent human fibroblast line. No age-dependent reduction in cytochrome oxidase specific activity was found in “senescent” human fibroblasts, nor was the specific activity of virus-transformed human fibroblasts significantly different than that of non-transformed cells. These data argue against changes in mitochondrial content as the basis for the limited in vitro lifespan of normal human fibroblasts, or for the cell line permanence of virus-transformed human fibroblasts.