The Effect of Calcium on the Fluidity and Phase Properties of Microsomal Membranes Isolated from Postclimacteric Golden Delicious Apples

Abstract

Calcium and magnesium have been found to stabilize and preserve the ethylene-synthesizing capacity of postclimacteric Golden Delicious apple slices. In order to assess if this reflects an effect at the membrane level, we have used three spin labels to assay changes in the physical properties of isolated microsomal membranes treated with calcium. A surface spin label, 18NP (an 18 carbon alkane with a terminal quaternary amine attached to a nitroxyl-containing pyrollidine ring), reported a 20.8% increase in rotational correlation time (τc) at 25°C and a 25.3% increase in activation energy (Ea) calculated from linear Arrhenius plots of τc for microsomal membranes treated with 50 mM CaCl2. Regions deeper within the bilayer were probed using two fatty acid spin labels—I(l2,3), stearic acid bearing a paramagnetic nitroxide group on carbon 5, and I(1,14), stearic acid bearing a paramagnetic nitroxide group on carbon 16. The calcium effect was less pronounced deeper within the lipid bilayer, there being only a 10.6% increase in τc and a 6% increase in the value of an order parameter (S) calculated from spectra recorded at 25°C for microsomes treated with 50 mM CaCl2 and labelled with I(1,14) and I(12,3) respectively. There was no significant change in Ea for I(1,14)-labelled membranes treated with 50 mM CaCl2. Similar trends were observed for membranes treated with 5 mM CaCl2, although the degree of change was less and differences were not always significant. Wide-angle X-ray diffraction revealed that untreated microsomal membranes from postclimacteric apples were exclusively liquid-crystalline at 25°C, as were microsomes treated with 50 mM CaCl2. The transition temperature, defined as the highest temperature at which gel phase lipid could be detected, was below −25°C for both treated and untreated membranes. It is apparent, therefore, that Ca2$ rigidifies and stabilizes membranes, particularly at their surfaces. Inasmuch as parts of the ethylene biosynthetic pathway appear to be membrane-associated, this rigidification may, in part, act to preserve ethylene production.