Content and binding characteristics of the mitochondrial ATPase inhibitor, IF1 in the tissues of several slow and fast heart-rate homeothermic species and in two poikilotherms
We determined the IF1 contents of pig, rabbit, rat, mouse, guinea pig, pigeon, turtle, and frog heart mitochondria and the effects of varying ionic strength upon the IF1-mediated inhibition of the ATPase activity of IF1-depleted rabbit heart mitochondrial particles (RHMP) by IF1-containing extracts from these same eight species. The IF1 binding experiments were run at both species-endogenous IF1 levels and at an IF1 level normalized to that present in rabbit heart mitochondria. When species-endogenous levels of rabbit heart IF1 or either speciesendogenous or normalized levels of pig heart IF1 were incubated with RHMP over a range of KCl concentrations, increasing the [KCl] to 150 mM had relatively little effect on IF1-mediated ATPase inhibition. When either species-endogenous or normalized levels of guinea pig, pigeon, turtle, or frog heart IF1 were incubated with RHMP under the same conditions, increasing [KCl] to 150 mM nearly completely blocked IF1-mediated ATPase inhibition. While species-endogenous levels of rat and mouse heart IF1 inhibited the ATPase activity of RHMP virtually not at all at any [KCl] examined, normalized levels of rat and mouse IF1 inhibited the ATPase activity of RHMP to the same extents as species-endogenous levels of pig and rabbit heart IF1, respectively, in the presence of increasing [KCl]. These experiments suggest that, while pig and rabbit heart mitochondria contain a full complement of higher-affinity IF1, pigeon, guinea pig, turtle, and frog heart mitochondria cell contain essentially a full complement of a lower-affinity form of IF1. In contrast, rat and mouse heart mitochondria contain only low levels of IF1 which exhibit binding characteristics similar to those of the pig and rabbit heart inhibitor. The guinea pig is the only mammal thus far examined that contains a loweraffinity form of IF1. In the present study we also determined the IF1 contents and IF1-to-F1 ATPase activity ratios of cardiac muscle, skeletal muscle, liver, and brain mitochondria of rabbit, pigeon, and rat, species representative of the three homeothermic regulatory classes.
Key wordsMitochondrial ATPase ATPase inhibitor protein IF1 myocardial ischemia effects of ionic strength higher and lower affinity IF1 homeothermic and poikilothermic species tissue distribution of If1 cardiac muscle skeletal muscle liver brain
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