Differential inhibition of respiration and its dependent H+ extrusion by fluorescamine in rat liver mitochondria

Slip and leak in mitochondrial oxidative phosphorylation

During oxidative phosphorylation by mammalian mitochondria part of the free energy stored in reduced substrates is dissipated and energy is released as heat. Here I review the mechanisms and the physiological significance of this phenomenon.

Amine fluorescamine compounds inhibit oxidative phosphorylation in rat liver mitochondria

The reaction of fluorescamine with ammonia, benzylamine, o,p-dimethylbenzylamine, 2-phenylethylamine, p-aminobenzoic acid, and the mycosamine-containing macrolide antibiotic, amphotericin B, yield compounds which induce significant effects on mitochondrial activities. From their effects on energy-yielding processes which lead to transmembranous proton movements, the compounds may be divided into three classes. While all modifiers significantly inhibit proton movement induced by both ATP hydrolysis and electron transfer in mitochondria, their influence on the primary energy yielding steps are quite different. Class I modifiers, e.g., the compound made from amphotericin B, inhibit electron transfer but have no effect on the Pi release associated with ATP hydrolysis. Class II modifiers, e.g., the compound made from benzylamine, inhibit respiration but stimulate Pi release. Class III modifiers, e.g., the compound made from p-aminobenzoic acid, on the other hand, only slightly increase Pi release but have no effect on redox reactions. These and other effects of the modifiers are taken to mean that the proton movements and their associated energy-yielding processes are only linked indirectly. The effects of the modifiers on State 3 mitochondrial activities were also investigated. Although all the modifiers decrease the rates of both State 3 respiration and its coupled ATP synthesis, the efficiency of energy conversion measured by the P/O ratio remains unaltered.

Chemical modification of the mitochondrial bc1 complex by N,N'-dicyclohexylcarbodiimide inhibits proton translocation

We report here that N,N'-dicyclohexylcarbodiimide (DCCD) decreases the H/2e stoichiometry of the cytochrome bc1 complex from 3.8 +/- 0.2 (10) to 2.1 +/- 0.1 (8) but has only a minimal effect on the H/2e ratio of cytochrome oxidase under the relatively mild conditions used. The effect on the bc1 complex cannot be explained by uncoupling, by inhibition of electron transport or by selective mitochondrial damage. We conclude that DCCD is an inhibitor of proton translocation within the bc1 complex. There are three possible explanations of this effect: (a) DCCD could alter the pathway of electron flow, (b) DCCD could prevent one of the proton translocation reactions but not electron transport, (c) DCCD could prevent the conduction of the translocated proton to the external phase.

Effects of borohydride-treated oligomycins on processes of energy transduction in mitochondria

Sodium borohydride in ethanol solution under mild conditions brings about the stepwise reduction of the 7-keto and the 11-keto groups of rutamycin and the oligomycins to the corresponding hydroxyl groups without further alterations of the macrocyclic lactone structure or other features of the molecule. The reduced compounds, as well as the parent antibiotics, inhibit the ADP-dependent (state 3) respiration, and the Pi formation and proton extrusion that are linked to ATP hydrolysis, but have no effect on other respiration-linked activities in intact rat liver mitochondria. Analogous inhibitory effects of borohydride-treated antibiotics are also observed in rat-liver submitochondrial particles. The reduced compounds are less potent inhibitors than the parent antibiotics. The reduced compounds are more efficient as inhibitors of Pi formation stimulated by conventional uncouplers (e.g. 2,4-dinitrophenol), than of Pi formation stimulated by certain amine-fluorescamine modifiers (e.g.) the benzylamine-fluorescamine compound. In contrast, the parent antibiotics are unable to discriminate between uncoupler-stimulated and modifier-stimulated Pi formation. It is suggested that rutamycin and the oligomycins bind to H+-ATPase as a result of hydrogen bonding to, at least, the 7-keto and/or the 11-keto groups of the antibiotics. When these keto groups are reduced to hydroxyl groups the hydrogen-bonding is less efficient due to the pronounced directional characteristic of hydrogen-bonding to keto groups.

Inhibition of the links between electron transfer and proton translocation in mitochondria

The mechanism by which proton extrusion is linked to electron transfer in mitochondria was investigated by means of the primary amine-specific reagent fluorescamine, and of compounds obtained from the reaction of fluorescamine with simple amines (e.g. benzylamine) and with the mycosamine-containing antibiotic amphotericin B. The effect of these 'modifiers' (i.e. fluorescamine transfer chain were assayed separately using specific inhibitors to block the action associated with the other site. Both types of modifiers inhibited the proton extrusion across the membrane to a significantly greater extent than the electron transfer process in both sites II and III. In contrast, the lactone derivative (or cyclic form) of the amine-fluorescamine compounds had no significant inhibitory effect on the proton extrusion and its associated electron transfer. These results are consistent with the hypothesis that the link between proton extrusion and electron transfer in mitochondria is indirect in nature. The results show that: (a) the links involved in sites II and III are identical or very similar in nature; (b) a covalent modification of primary amino groups in the inner membrane is not essential for the expression of these differential inhibitory effects; (c) specific structural features in the amine-fluorescamine compounds, and in the mitochondria-fluorescamine derivatives, are crucial for the expression of the inhibitory effects. Our results contradict the 'redox loop' model of Mitchell, and are compatible with the proton pump concept for the linked proton translocation in oxidative phosphorylation.