Functional interactions of lipids and proteins in rat intestinal microvillus membranes

Abstract

Interactions of lipids and proteins in isolated rat intestinal microvillus membranes were examined by studying the temperature dependence of enzyme activities and of D-glucose transport in relation to the membrane lipid thermotropic transition observed by fluorescence polarization (26 .+-. 2.degree. C) and differential scanning calorimetry (23-39.degree. C). Two groups of activities were defined. Enzymes of the 1st group, comprising lactase, maltase, sucrase, leucine aminopeptidase and .gamma.-glutamyl transpeptidase, all yielded a single slope on the Arrhenius plot in the range 10-40.degree. C and did not appear to experience functionally the effects of the lipid thermotropic transition. Each activity of the 2nd group, comprising Ca and Mg-dependent ATPases, p-nitrophenylphosphatase, and D-glucose transport, showed a change in the slope of the Arrhenius plot in the range 25-30.degree. C, corresponding to the lower region of the lipid transition. The terms extrinsic and intrinsic activities could be applied to these groups. Delipidation of the particulate p-nitrophenylphosphatase removed the discontinuity in the Arrhenius plot. Subsequent relipidation with a variety of lipids restored a break point, but the temperature corresponded to the original discontinuity (25-29.degree. C) rather than to the phase transition temperature of the exogenous lipid added.