Watras J, Messineo F C, Herbette L G
J Biol Chem. 1984 Jan 25;259(2):1319-24.
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.
为阐明先前报道的在棕榈酸存在下肌浆网囊泡钙螯合增加的机制，采用钙指示剂偶氮胂III通过双波长分光光度法以及使用45CaCl2通过密理博过滤法研究了这种脂肪酸结合钙的能力。在120 mM KCl、1 mM MgCl2、15 μM CaCl2和40 mM组氨酸（pH 6.8，25℃）存在的情况下，钙与10 - 80 μM棕榈酸的结合缓慢（半衰期约3 - 7分钟），并伴随着浊度增加（通过差示分光光度法测量）和可交换钙减少。在此条件下，钙与棕榈酸结合的化学计量比为0.15摩尔钙/摩尔棕榈酸酯，但在0.03 - 3 mM钙存在时增加到约0.4摩尔钙/摩尔棕榈酸酯。钙与棕榈酸的结合速率也随钙浓度增加，使得在1 - 3 mM钙存在时，棕榈酸在15秒内结合约0.4摩尔钙/摩尔棕榈酸酯。因此，棕榈酸可能在细胞内钙浓度较高的区域（如肌浆网腔）代表一种钙沉淀阴离子。对存在不同棕榈酸浓度时肌浆网钙螯合的分析支持这一假设，并表明10%的棕榈酸可被腔内钙接触到。
