The Temperature Dependence of Respiration and ATPase in Rat Liver Mitochondria is Altered by Ethanol

Abstract

We studied hepatic mitochondria to determine the effects of ethanol in vitro and of chronic ethanol consumption on the temperature dependence (10°–45°C) of a) substrate oxidation, and b) ATP hydrolysis, with or without CCCP. Arrhenius plots showed the characteristic breaks around 20°C both for electron transport and ATP hydrolysis with high energy of activation at low temperature and low energy of activation at high temperature. Ethanol, in vitro generally lowered the energy of activation at high temperature and shifted the break in the Arrhenius plots to lower temperatures suggesting an increase in membrane fluidity. At 40°C and above ethanol accelerated electron transport and greatly stimulated ATPase activity. In mitochondria from ethanol-fed rats, Arrhenius plots showed a shift in the breaks to a higher temperature, a finding which suggests a change in membrane structure, possibly associated with decreased fluidity. This may be an adaption of the mitochondrial membranes to counter the effect of ethanol on membrane structure.

An important property of membrane enzymes which may be related to the physical state of the lipid membrane, is the temperature dependence of enzyme activity. A change in membrane structure is often associated with a change in the energy of activation of membrane-bound enzyme reactions (Overath et al., 1970). We have, therefore, attempted to study the physical properties of the mitochondrial inner membrane by investigating the temperature dependence of NADH-mediated substrate oxidation and ATPase activity in rat liver mitochondria. We report that both ethanol administration in vitro and chronic ethanol consumption have significant but opposite effects on the temperature dependence of mitochondrial ATPase and NADH-oxidase acti vi ty. In general, ethanol in vitro appears to lower the transition temperature in Arrhenius plots, a finding which is consistent with its fluidizing effect on biological membranes (Hill, 1974). By contrast, chronic ethanol consumption appears to raise the transition temperature, an observation which might indicate increased viscosity of the membranes. Overall, mitochondria are more susceptible to uncoupling effect of ethanol in vitro at high temperatures. However, at high temperatures, coupled respiration and coupled ATPase activities appear to be significantly less sensitive to ethanol in vitro in chronically treated animals than in controls.