RESUMO
The phosholipid bilayer fluidity of isolated mitochondria and phospholipid vesicles after calcium-dependent binding of annexin V was studied using EPR spectroscopy. The membranes were probed at different depths by alternatively using cardiolipin, phosphatidylcholine, or phosphatidylethanolamine spin labeled at position C-5 or C-12 or C-16 of the beta acyl chain. Computer-aided spectral titration facilitated observing and quantitating the EPR spectrum from phospholipid spin labels affected by annexin binding, and spectral mobility was calibrated by comparison with standard spectra scanned at various temperatures. In most cases it was found that binding of the protein to the membranes makes the inner bilayer more rigid up to acyl position C-12 than afterward, in agreement with the previously observed effect in SUVs [Megli, F. M., Selvaggi, M., Liemann, S., Quagliariello, E., and Huber, R. (1998) Biochemistry 37, 10540-10546]. Moreover, in isolated mitochondrial membranes, cardiolipin apparently is more readily affected than the other main phospholipids, while in vesicles made from mitochondrial phospholipids, the different species are affected in essentially the same way. This behavior is consistent with the existence of distinct cardiolipin pools in mitochondria, and with the already advanced hypothesis that these domains are the binding site for annexin V to the isolated organelles [Megli, F. M., Selvaggi, M., De Lisi, A., and Quagliariello, E. (1995) Biochim. Biophys. Acta 1236, 273-278]. Keeping in mind the funcional importance of cardiolipin in the mitochondrial membrane, the question is raised as to whether the observed influence of annexin V binding to this phospholipid and its consequent local fluidity alteration might affect the mitochondrial functionality, at least in vitro.
