title: Low reserve of cytochrome c oxidase capacity in vivo in the respiratory chain of a variety of human cell types.
abstract: The question of whether and to what extent the in vivo cytochrome c oxidase (COX) capacity in mammalian cells exceeds that required to support respiration is still unresolved. In the present work, to address this question, a newly developed approach for measuring the rate of COX activity, either as an isolated step or as a respiratory chain-integrated step, has been applied to a variety of human cell types, including several tumor-derived semidifferentiated cell lines, as well as specialized cells removed from the organism. KCN titration assays, carried out on intact uncoupled cells, have clearly shown that the COX capacity is in low excess (16-40%) with respect to that required to support the endogenous respiration rate. Furthermore, measurements of O2 consumption rate supported by 0.4 mM tetramethyl-p-phenylenediamine in antimycin-inhibited uncoupled intact cells have given results that are fully consistent with those obtained in the KCN titration experiments. Similarly, KCN titration assays on digitonin-permeabilized cells have revealed a COX capacity that is nearly limiting (7-22% excess) for ADP + glutamate/malate-dependent respiration. The present observations, therefore, substantiate the conclusion that the in vivo control of respiration by COX is much tighter than has been generally assumed on the basis of experiments carried out on isolated mitochondria. This conclusion has important implications for understanding the role of physiological or pathological factors in affecting the COX threshold.
authors.affiliation: Division of Biology, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: G
authors.initials: G
authors.lastname: Villani
authors.forename: M
authors.initials: M
authors.lastname: Greco
authors.forename: S
authors.initials: S
authors.lastname: Papa
authors.forename: G
authors.initials: G
authors.lastname: Attardi
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D006528:Carcinoma, Hepatocellular; D002454:Cell Differentiation; D002460:Cell Line; D002463:Cell Membrane Permeability; D004072:Digitonin / Q000494:pharmacology; D003576:Electron Transport Complex IV / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008113:Liver Neoplasms; D008175:Lung Neoplasms; D008293:Malates / Q000378:metabolism; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D009101:Multiple Myeloma; D009447:Neuroblastoma; D012516:Osteosarcoma; D010101:Oxygen Consumption / Q000187:drug effects; D011190:Potassium Cyanide / Q000494:pharmacology; D013772:Tetramethylphenylenediamine / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D008293:Malates; D018698:Glutamic Acid; D000244:Adenosine Diphosphate; D003576:Electron Transport Complex IV; D004072:Digitonin; D011190:Potassium Cyanide; D013772:Tetramethylphenylenediamine