It is claimed, though not without dispute, that genetically engineered mammalian cells grow more slowly than their progenitor
cells because the recombinant gene system causes a metabolic burden. This was found to be the case for CHO cells transfected
with expression vectors forcytochrome b5. The slower growth was associated with lower metabolic activity measured by heat flux and mitochondrial activity (rhodamine
123 fluorescence). The calorimetric-respirometric ratio was similar for all cell types, implying that the greater fluxes of
glucose and glutamine in the recombinant cells was channelled to biosynthesis. This demand probably restricted the supply
of pyruvate to the mitochondria in these cells.