Motion of cytochrome c bound to giant (2-10-micron diam) mitochondria isolated from the waterbug Lethocerus indicus was examined using the technique of fluorescence recovery after photobleaching. Fluorescent cytochrome c was exchanged for native cytochrome c through partly damaged outer membrane. Recovery profiles were not statistically different when the fluorescence from iron-free cytochrome c or fluorescein-labeled cytochrome c was used and were essentially the same in the presence or absence of an uncoupler. In the presence of excess porphyrin cytochrome c, the apparent diffusion coefficient was 6 X 10(-11) cm2/s in 0.3 M sucrose-mannitol-EDTA and 3 X 10(-10) cm2/s in 0.10 M KCl/0.10 M sucrose. At concentrations of porphyrin cytochrome c that are stoichiometric with cytochrome c oxidase and for mitochondria in which excess cytochrome c was washed away, two components were observed in the recovery profile. The diffusion coefficient of the fast component was 1 X 10(-10) cm2/s. The second component showed no recovery during the time scale of measurement (D less than 10(-12) cm2/s). We speculate on the origin of the immobile fraction.

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