Effect of antibiotics on respiration in human cells.

Abstract

Extract: Chloramphenicol (CAP), chloramphenicol succinate (CAP-S), tetracycline (TET), and erythromycin (EM) inhibited synthesis of a mitochondrial enzyme, cytochrome oxidase, in cultured human fibroblasts; the 50% inhibitory concentrations were 7.5, 13, 18, and 350 $mUM respectively, or 2.4, 4.1, 8, and 250 $mUg/ml. In fibroblasts grown in CAP-S, cytochrome oxidase declined exponentially with a half-life of approximately 2.0 days. Oxygen uptake, on the other hand, was unchanged for the first 24–48 hr of exposure to CAP-S, and then declined, but at a much slower rate than did cytochrome oxidase; after 7 days, cytochrome oxidase activity was 6% of the control value, whereas oxygen uptake was 50% of control. From the kinetic data we conclude that cytochrome oxidase is not normally rate-limiting in cellular respiration, but that, after a period of exposure to CAP-S, an enzyme whose synthesis is inhibited by CAP-S becomes rate-limiting. This enzyme is not cytochrome oxidase, but more likely another component of the respiratory chain. Both CAP and TET inhibited cytochrome oxidase accumulation in non-growing as well as in growing fibroblasts, with enzyme levels declining to 50% of control values in 48 hr; after that time, a new, lower steady state level of enzyme was attained. Oxygen uptake was not inhibited in fibroblasts by short (4-hr) exposure to CAP, CAP-S or TET in concentrations up to 1 mM. Speculation: Antibiotics which act by inhibiting bacterial protein synthesis also inhibit mitochondrial enzyme synthesis in virtually all eukaryotic organisms and cells. The rarity of obvious clinical sequellae in patients receiving these agents is explained by the fact that cytochrome oxidase and other components of the respiratory chain, the synthesis of which is inhibited by these agents, are not normally rate-limiting in cellular respiration. After prolonged exposure to these antibiotics, a rate-limiting level of a component of the respiratory chain is reached, but its activity declines slowly and the restriction in respiratory capacity is consequently modest.