Proton permeability

In rat liver mitochondria, in state 4, the AP was estimated to be about 220 mV, with the membrane potential representing about 90% of this (Nicholls, 1974 Appendix 3). Similar values have been reported for human and rat skeletal muscle mitochondria in state 4 (Stumpf et al., 1982). The control of the rate of electron transport is not only determined by the availability of ADP, but also of Pj oxidizable substrates, and oxygen. There is evidence for futile cycling of protons in intact normal rat hepatocytes (Brand et al., 1993). Recently, Porter and Brand (1993) found a correlation between the proton permeability of the inner membrane of liver mitochondria and body size in animals from the mouse (20 g) to horses (150 kg) with a decrease in permeability with increasing weight of several-fold at a constant... 

ARTIFICIAL PHYTANYL-CHAINED GLYCOLIPID VESICLE MEMBRANES WITH LOW PROTON PERMEABILITY ARE SUITABLE FOR PROTON PUMP RECONSTITUTION MATRICES... 

DeamerDW. 1990. Anesthetic eiJects on membrane proton permeability. USDA/CRIS database. July 1990. 

Introduction of HM mixture induced passive permeability of mitochondria membranes for K, and resulted in aggravation of proton permeability (Fig. 22.3). Statistically significant differences in swelling rate of mitochondria isolated from the rat liver between different animal groups, was observed in ammonium nitrate medium from the second minute tiU the end of the experiment. Similar dynamics... 

Palacios, J. and Serrano, R. (1978). Proton permeability induced by polyene antibiotics. Aplausible mechanism for their inhibition of maltose fermentation in yealsEBS Lett, 91,198-201. 

Brookes, P.S., J.A. Buckingham, A.M. Tenreiro, A.J. Hulbert, and M.D. Brand (1998). The proton permeability of the inner membrane of liver mitochondria from ectothermic and endothermic vertebrates and from obese rats correlations... 

V. V. Petrov and L. A. Okorokov (1990). Increase of the anion and proton permeability of Saccharomyces carlsbergensis plasmalemma by n-alcohols as a possible cause of its de-energization. Yeast, 6, 311-318. 

Blume A, Lipid phase transitions water and proton permeability. 

The energy-transducing membrane is topologically closed and has a low proton permeability... 

Increased acid tolerance correlates strongly with a decrease in proton accumulation in the cytoplasm. This altered proton permeability is again associated with changes in the protein composition of the cell membranes. Acid-adapted E. coli changes the lipid composition of its membranes, and elevated levels of cyclopropane fatty acids are often found. This may mean that changes in the protein composition of the cell membrane are a result of changes in the membrane lipid composition. Both lipid and protein alterations may be necessary to protect a bacterial cell in acidic environments (Jordan, Oxford, and O Byrne, 1999). 

The rate of proton recombination has been measured either from the time-resolved kinetics or using the steady-state Equation (5). The values calculated by both methods are shown in Figure 22, which relates the rate of the reaction, as calculated by Equation (21) with the radius of the proton-permeable space. The experimental results cluster on the theoretical curve in the range of R = 72 7 A, a good approximation with the internal radius of a small liposome (Brauillette et al., 1982). 

The effect of k JkOM on the dynamics of pulse protonation of class-I indicator is depicted in Figure 39. A low ratio implies that the In(c) population is small, and there will be no apparent difference between Ks and Kobs (see Equation 57), which is the case for Neutral Red. In such cases, there will be no kinetic abberrations due to the interphase transition. As the ratio km/kOM increases, the contribution of In(c) population will increase, affecting all measured macroscopic parameters. At a very high ratio, the In(s) population is becoming so small that the pulse protonation will decline to nearly unobservable magnitude. This is the most profound distinction between the dynamic properties of two classes. Class-I I compounds will participate in pulse protonation whatever the kiJkout ratio, as their proton acceptor form is located in the proton-permeable phase. On the other hand, class-I compounds may be totally masked in pulse experiment due to depletion of the basic form from the (s) phase. 

Under these conditions, a typical measured proton flux might be in the range of 10 15 mol/(cm2 s). To compare this value with that of potassium, 1 M potassium ion (as potassium sulfate) could be trapped inside the same liposomes, and potassium efflux into 1 M choline sulfate could be measured with a potassium-sensitive electrode. A typical result might again be in the range of 10 15 mol/(cm2 s). The proton permeability anomaly now becomes clear The same flux is measured for both potassium ion and protons, yet the proton flux is driven by a concentration of protons 6 orders of magnitude less than the concentration of potassium ions. Estimates of the relative permeabilities of the bilayer to protons and potassium using these flux data yield values of 10 6 cm/s for protons and 10 12 cm/s for potassium ion. 

Proton permeability can now be compared with the permeability of other cations in lipid bilayer systems, specifically liposomes (Table I) (15-21). First, it is important to note that the intrinsic proton permeability of a given lipid bilayer depends on its physical state (gel or fluid) and the size of the vesicles. This dependence means that the proton permeability of lipid bilayers can vary by as much as 3 orders of magnitude when small vesicles composed of relatively saturated lipid (P = 10 7 cm/s) are compared to large vesicles composed of highly unsaturated lipid (P = 10 4 cm/s). The permeability is not strongly dependent on the lipid head group. 

The high permeability, however, is balanced by the very low concentration that drives proton flux in typical membranes, so the conductance is low. This situation permits coupling membranes like those of mitochondria to maintain proton gradients even though proton permeability is high. 

The proton permeability is affected by lipid composition and vesicle size over about 3 orders of magnitude. 

Temporal proton-wire (a transient hydrogen-bonded chain of water molecules) formahon through the lipid bilayer may be one of the most plausible explanations of abnormally high proton permeability [61, 62, 77] In this hypothesis, the proton is transferred through the hydrogen-bond network that the proton wire has formed. 

---