Cation-induced changes in the membrane fluidity of isolated corn mitochondria as determined by nitroxide spin labels

Cooke, A., Collison, D, Mabbs, F. E. and Earnshaw, M. J. 1985. Cation-induced changes in the membrane fluidity of isolated corn mitochondria as determined by nitroxide spin labels.—J. exp Bot. 36: 1799–1808. The addition of Ca2− or La3+ to non-energized corn mitochondria, with incorporated spin labels, results in an increase in 2Tu of the membrane surface label I (12, 3) and an increase in of the membrane core label 1(1, 14). These results indicate a decrease in the motion of the label within the mitochondrial membranes. Decreasing the temperature also increases the 2Tu and torque; of I (12, 3)- and I (1, 14)-labelled corn mitochondria respectively. This suggests that a fall in temperature acts similarly to the addition of cations in that the freedom of motion of spin labels in the membrane is limited. Comparing the temperature-induced changes in label motion to those of Ca2+ implies that the membrane core is more sensitive to Ca2+ -induced changes in motion than is the surface. A survey of a range of multivalent cations suggests that their effect on spin label motion is largely non-specific and probably due to cation binding.