Mechanisms of fatty acid effects on sarcoplasmic reticulum. I. Calcium-fatty acid interaction

To elucidate the mechanism for the previously reported increase in calcium sequestration by sarcoplasmic reticulum vesicles in the presence of palmitic acid, the ability of this fatty acid to bind calcium was investigated by dual wavelength spectrophotometry using the calcium indicator arsenazo III,, and by Millipore filtration using 45CaCl2. In the presence of 120 mM KCl, 1 mM MgCl2, 15 microM CaCl2, and 40 mM histidine (pH 6.8, 25 degrees C), calcium binding to 10-80 microM palmitic acid occurred slowly (approximately 3-7 min halftime) and was accompanied by an increase in turbidity (measured by difference spectrophotometry) and a decrease in exchangeable calcium. The stoichiometry of calcium binding to palmitic acid under this condition was 0.15 mol of calcium/mol of palmitate, but increased to approximately 0.4 mol of calcium/mol of palmitate in the presence of 0.03-3 mM calcium. The rate of calcium binding to palmitic acid also increased with calcium concentration such that in the presence of 1-3 mM calcium, palmitic acid bound approximately 0.4 mol of calcium/mol of palmitate within 15 s. Thus, palmitic acid may represent a calcium-precipitating anion in regions of the cell, such as the lumen of the sarcoplasmic reticulum, where calcium concentration is high. Analyses of sarcoplasmic reticulum calcium sequestration in the presence of various palmitic acid concentrations support this hypothesis and suggest that 10% of the palmitic acid is accessible to luminal calcium.