Apo C-IIのLipoprotein lipase活性化機構

Abstract

The effect of apolipoprotein C-II (apo C-II) on the bovine milk lipoprotein lipase (LPL)-catalysed hydrolysis of a homogeneous series of saturated phosphatidylcholines (PC) was examined with respect to the fatty acyl chain length of the substrates. Dilauryl-, dimyristoyl-, dipalmitoyl-, and distearoylphophatidylcholine (diC12PC, diC14PC, diC16PC and diC18PC, respectively) solubilized by Triton X-100 were used as substrates. The maximal rate of the LPL-catalysed hydrolysis of each of these lipids was determined in the absence and presence of apo C-II. The activation factor (the ratio of enzyme activity with apo C-II to that without the activator protein) increased with increasing mol ratio of apo C-II to LPL and was maximal at a ratio of about 50. At all apo C-II/ LPL mol ratio tested, the activation factor increased as a function of fatty acyl chain length.When LPL activities were determined between 16C and 40C, no systematic relationship between substrate fatty acyl chain length and either the rates or the activation energies for hydrolysis in the presence or absence of apo C-II was observed. However, there was a linear relationship between fatty acyl chain length and both the logarithm of the activation factor and the difference in activation energy in the presence and absence of apo C-II.These relationship were not the result of an alteration in the physical form of the substrate, since a mixture of diC14PC and diC16PC gave activation factors for each PC which were the same as those obtained for each individual lipid. From the temperature dependence of the activation factor, thermodynamic functions of the apo C-II-induced change in the reaction pathway were calculated. The free energy of activation decreased linearly with increasing chain length as the result of a linear increase in activation entropy which more than offset the unfavorable increase in activation enthalpy. We propose that the apo C-II mediated increase in the rate of LPL-catalysed hydrolysis of phosphatidylcholine is associated with transfer of a fatty acyl chain of the substrate or product to a more hydrophobic environment within the transition state complex.