In this paper, we shall review various antioxygen defense systems of the cell paying particular attention to those that prevent superoxide formation rather than scavenge already formed superoxide and its products. The role of uncoupled, decoupled and non-coupled respiration, mitochondrial pore, mitochondrion-linked apoptosis will be considered. Mitochondrial theory of aging will be regarded in context of reactive oxygen species-induced damage of mitochondrial DNA.
A novel intermediate (P) of the bacteriorhodopsin (bR) photocycle, appearing between M412 and bR is described. Like bR, intermediate P shows an absorption maximum at 56&570 nm. However, the extinction coefficient of P is somewhat lower than that of bR. Moreover, there are some differences in spectra of bR and P at wavelengths shorter than 450 nm. The P+bR transition correlates with the absorption of H+ from the water medium. The following conditions proved to be favourable for the detection of the new intermediate: a high salt concentration, low light intensity and low temperature (0.5"C). The P+bR transition is strongly decelerated by a small amount of Triton X-100. Illumination of P does not produce M412 before bR is formed. It is assumed that M412 converts to P when the Schiff base is protonated by a proton transferred from a protein protolytic group which participates in the inward H+-conductivity pathway. Reprotonation of this group results in the conversion of P to bR. No more than 1 H+ is transported per bR photocycle.
Regeneration of bacteriorhodopsin (bR) from the M intermediate via a new intermediate P has been studied. In the purple sheets treated with 0.015% Triton X-100 (the P -+ bR transition is suppressed), the cc-maximum of absorption of P is located at 560 + 5 nm, the extinction coefficient being equal to 0.7 f 0.1 of that of bR. Besides, there is a small but measurable absorbance increase at 33&350 nm, which seems to be due to a /3-maximum of 13-cis-retinal residue in P. The similarity of the cc-maximum of P and bR suggests the Schiff base nitrogen to be protonated at the M + P transition. The kinetics of P + bR transition measured at the 335 nm absorbance decrease coincides with that of proton uptake accompanying bR regeneration after flash. A scheme is proposed assuming that Ht absorption from the water phase is coupled to the 13-cis + all-trans isomerization of retinal residue in which the Schiff base is already protonated by a proton-donor group of the protein.
Oxidation of semiquinone by O2 in the Q cycle is known to be one of the sources of superoxide anion (O2.-) in aerobic cells. In this paper, such a phenomenon was analyzed using the chemical kinetics model of electron transfer from succinate to cytochrome c, including coenzyme Q, the complex III non-heme iron protein FeSIII and cytochromes bl, bh and cl. Electron transfers from QH2 to FeSIII and cytochrome bl were assumed to occur according to direct transfer mechanism (dynamic channelling) involving the formation of FeS(red)III-Q.- and Q.--cytochrome bl complexes. For oxidation/reduction reactions involving cytochromes bh and bl, the dependence of the equilibrium and elementary rate constants on the membrane potential (deltapsi) was taken into consideration. The rate of O2.- generation was found to increase dramatically with increase in deltapsi above the values found in State 3. On the other hand, the rate of cytochrome c reduction decreased sharply at the same values of the membrane potential. This explains experimental data that the O2.- generation at State 4 appears to be very much faster than at State 3. A mild uncoupling in State 4 can markedly decrease the superoxide generation due to a decrease in deltapsi below the above mentioned critical level. DeltapH appears to be equally effective as deltapsi in stimulation of superoxide production which depends, in fact, upon the deltamuH+ level.
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