Weinberg R B, Spector M S
J Biol Chem. 1985 Apr 25;260(8):4914-21.
The in vivo affinity of human apolipoprotein A-IV (apo-A-IV) for plasma lipoproteins is considerably less than that of other apolipoproteins. We have therefore studied its spectroscopic properties and its association with model chylomicrons to investigate its structural characteristics and to define their influence upon its affinity for lipids. Fluorescence emission spectra of apo-A-IV in dilute aqueous solution revealed that its single tryptophan residue resides in a pH-sensitive hydrophobic domain, which is maximally protected from iodide quenching at pH 7.5. Denaturation of apo-A-IV by guanidine hydrochloride caused a multiphasic fluorescence emission red shift, with an unusual enhancement of quantum yield. Circular dichroism spectroscopy of apo-A-IV demonstrated negative ellipticity maxima at 210 and 222 nm, consistent with 54% alpha-helical structure. The alpha-helicity of apo-A-IV as measured by [theta]222 was also pH-sensitive and displayed a distinctive decrease between pH 7.0 and 8.0. Apo-A-IV was exquisitely sensitive to denaturation by guanidine hydrochloride, and its estimated free energy of stabilization in aqueous solution was near zero. Apo-A-IV bound to the surface of Sf greater than 400 particles of a phospholipid-triglyceride emulsion in a noncooperative, concentration-dependent manner. The affinity of apo-A-IV for these model chylomicrons was influenced by changes in pH or addition of guanidine hydrochloride in a manner which correlated well with the structural changes observed under similar conditions. We conclude that human apolipoprotein A-IV possesses several biophysical properties characteristic of the better studied plasma apolipoproteins, yet, apo-A-IV appears to be marginally stable in aqueous solution and its structural characteristics and lipid binding properties are particularly sensitive to environment.
人载脂蛋白A-IV（apo-A-IV）在体内对血浆脂蛋白的亲和力远低于其他载脂蛋白。因此，我们研究了它的光谱特性及其与模型乳糜微粒的结合，以研究其结构特征，并确定这些特征对其脂质亲和力的影响。稀水溶液中apo-A-IV的荧光发射光谱表明，其单一色氨酸残基位于一个对pH敏感的疏水区，在pH 7.5时该区域受到碘化物淬灭的保护最大。盐酸胍对apo-A-IV的变性导致多相荧光发射红移，量子产率异常增加。apo-A-IV的圆二色光谱在210和222 nm处显示负椭圆率最大值，与54%的α-螺旋结构一致。通过[θ]222测量的apo-A-IV的α-螺旋度也对pH敏感，在pH 7.0和8.0之间有明显下降。apo-A-IV对盐酸胍变性极为敏感，其在水溶液中的估计稳定自由能接近零。apo-A-IV以非协同、浓度依赖的方式与大于400的磷脂-甘油三酯乳液的Sf颗粒表面结合。apo-A-IV对这些模型乳糜微粒的亲和力受pH变化或盐酸胍添加的影响，其方式与在类似条件下观察到的结构变化密切相关。我们得出结论，人载脂蛋白A-IV具有一些已被深入研究的血浆载脂蛋白所特有的生物物理特性，然而，apo-A-IV在水溶液中似乎仅具有边际稳定性，其结构特征和脂质结合特性对环境特别敏感。
