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 isolatedcorn mitochondria as determined by nitroxide spin labels.—J.exp Bot. 36: 1799–1808. The addition of Ca^2– or La³⁺ to non-energized corn mitochondria,with incorporated spin labels, results in an increase in 2T_uof the membrane surface label I (12, 3) and an increase in ofthe membrane core label 1(1, 14). These results indicate a decreasein the motion of the label within the mitochondrial membranes. Decreasing the temperature also increases the 2T_u and torque;of I (12, 3)- and I (1, 14)-labelled corn mitochondria respectively.This suggests that a fall in temperature acts similarly to theaddition of cations in that the freedom of motion of spin labelsin the membrane is limited. Comparing the temperature-inducedchanges in label motion to those of Ca²⁺ implies that the membranecore is more sensitive to Ca²⁺ -induced changes in motion thanis the surface. A survey of a range of multivalent cations suggests that theireffect on spin label motion is largely non-specific and probablydue to cation binding. Key words: Calcium, mitochondria, membranes, fluidity