Electron transfer from excited tryptophan to cytochrome c: mechanism of phosphorescence quenching?
Parvalbumin, aldolase and liver alcohol dehydrogenase (ADH), proteins exhibiting long-lived phosphorescence lifetimes at room temperature, were examined for their reactivity with ferricytochrome c (cytochrome c Fe3+) as an external electron acceptor. Illumination of a reaction mixture containing protein and cytochrome c in the absence of oxygen brought about reduction of cytochrome c in relation to the duration of light. The largest portion of reduced cytochrome c was found with a sample containing ADH, where a 50% reduction of cytochrome c was reached after 5 min of illumination with a xenon lamp. Parvalbumin and aldolase were about half as effective under the same conditions. Several lines of evidence support the idea that the reaction of cytochrome c occurred by a long-range electron transfer from the excited triplet state of tryptophan. First, cytochrome c quenches the tryptophan phosphorescence and with parvalbumin, its bimolecular quenching rate constant, kq, was 2.9 x 10(6) M-1 s-1. Second, when the illuminated reaction mixture was supplied with 0.2 mM to 1 mM nitrite, a concentration range of nitrite which quenches the tryptophan phosphorescence but not the fluorescence, the amount of reduced cytochrome c on illumination markedly decreased. Finally, for all illuminated protein samples, the extent of cytochrome c reduction occurred parallel to a decrease in tryptophan content as judged from a decrease in fluorescence intensity and/or a decrease in tryptophan absorption at 280 nm.
Dadak
V
V
Department of Biochemistry, Faculty of Science, Masaryk University, Brno, Czechoslovakia.
Vanderkooi
J M
JM
Wright
W W
WW
D000426:Alcohol Dehydrogenase / Q000378:metabolism; D000818:Animals; D003574:Cytochrome c Group / Q000378:metabolism*; D004579:Electron Transport; D008163:Luminescent Measurements; D010084:Oxidation-Reduction; D010320:Parvalbumins / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000378:metabolism*; D014466:Ultraviolet Rays
D003574:Cytochrome c Group; D010320:Parvalbumins; D014364:Tryptophan; D000426:Alcohol Dehydrogenase