Abstract
Rat intestinal brush border membranes (BBM) were prepared by discontinuous sucrose gradient centrifugation. This specific fraction contained alkaline phosphatase activity. A dramatic decrease in the specific activity of the BBM-bound alkaline phosphatase was observed at different concentrations of Ca 2+ and vitamin D 3. Studies of the temperature dependence (5–40°) of alkaline phosphatase reveal a change in energy of activation (slope of the Arrhenius plot) at 23.0 ± 1.1° which was elevated to 27.8 ± 1.3° in BBM treated with Ca 2+, while it was depressed to 17.2 ± 1.2° in BBM treated with vitamin D 3. Membrane lipid fluidity, as assessed by the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), was significantly greater in BBM treated with vitamin D 3 and significantly lower in Ca 2+ treated BBM. A lipid thermotropic transition temperature was observed at 22.2 ± 1.2° which rose to 28.3 ± 1.4° in Ca 2+ treated BBM and reduced to 17.0 ± 1.2° in vitamin D 3 treated BBM. The biological significance of these results is discussed in terms of modifications in the lipid-protein interactions in BBM, induced by Ca 2+ and vitamin D 3, and the implications in the physiological intestinal transport processes of calcium.
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