Reaction of bulk protons with a mitochondrial inner membrane preparation: time-resolved measurements and their...

The laser-induced proton pulse technique [Gutman, M. (1986) Methods Enzymol. 127, 522-538] was applied on suspensions of submitochondrial vesicles, and the exchange of protons between the bulk and the mitochondrial membranes was measured in the time-resolved domain with a submicrosecond resolution. The protons were discharged by photoexcitation of pyranine (8-hydroxypyrene-1,3,6-trisulfonate) by a short laser pulse, and the reprotonation of the pyranine anion was monitored at 457.8 nm. In parallel, the protonation of the membrane was followed at 496.5 nm, looking at the transient absorbance of fluorescein, covalently attached to the M side of the membrane. The analysis of the relaxation dynamics was carried out by a simulation procedure that reconstructs the observed dynamics of the two chromophores. The analysis revealed the presence of the membrane indigenous buffering moieties. The low-pK buffer (pK 4.1) was present in a quantity of 100 +/- 20 nmol/mg of protein, and its kinetics indicate that it appears in multianionic clusters bearing a negative electric charge. The medium-pK buffer (pK 6.9) was present in a larger quantity (200 +/- 20 nmol/mg), and its kinetic parameter indicated clustering into positively charged domains. Both types of indigenous buffer reacted with the proton and pyranine anion in unhindered diffusion-controlled reactions. On the other hand, the exchange of protons between the indigenous buffer moieties was rather slow. No evidence was found for the presence of sites capable of retaining a proton, secluded from the bulk, for a time frame longer than 100 microseconds as required by the models of localized proton gradient.