Calcium Modulates Fatty Acid Dynamics in Rat Liver Plasma Membranes

Modulation of free fatty acid binding in isolated rat liver plasma membrances was evaluated using the fluorecent fatty acids trans ‐parinaric and cis ‐parinaric acid as analogues for saturated and unsaturated fatty acids, respectigvely. Binding of trans ‐parinarate but not cis ‐parinarate was inhibited by physiological levels of Ca 2+ . The effect was revedrsed by addition of excess EGTA. Calcium decreased the aqueous to lipid partition coefficient, K p , of trans ‐parinaric acid for liver plasma membrances while increasing the K p for trans ‐parinaric acid. In addition, Ca 2+ also altered the fluorescence lifetime, the quantum yield, and the relative partitioning of trans ‐parinaric and cis ‐parinaric acid into fluid and solid phases. Calcium and EGTA did not affect the binding of 1, 6‐diphenyl‐1,3,5‐hexatriene. The effect of Ca 2+ on the liver plasma membrane structure was to increase the rigidity of the membrane, primarily the solid domain. The fluorescence polarization of trans ‐parinarate, cis ‐parinarate, and 1,6‐diphenyl‐1,3,5‐hexatriene at 24°C in liver plasma membranes in the absence of Ca 2+ was 0.295 ± 0.008, 0.253 ± 0.007, and 0.284 ± 0.005, respectively. Calcium (2.4 mM) increased the polarization of these probe molecules in liver plasma membrances by 8–10%. EGTA (3.4 mM) reversed or abolished the increase in polarization. Thus, the fluorescent fatty acids trans ‐parinarate and cis ‐parinarate may be used to monitor fatty acid binding by isolated membranes, to evaluate factors such as Ca 2+ which modulate fatty acid binding, and to investigate the microenvironemtn in which the fatty acids reside. The data suggest that Ca 2+ may be an important regulator of fatty acid uptake by the liver plasma membrane, and therby interact with intermediary metabolism of lipids at a step not involging lipolytic or synthetic enzymes.