Ryan R O, Kay C M, Oikawa K, Liu H, Bradley R, Scraba D G
Department of Biochemistry, University of Alberta, Edmonton, Canada.
J Lipid Res. 1992 Jan;33(1):55-63.
Four distinct subspecies of the major insect lipoprotein, lipophorin, that range in overall lipid content from 20 to 51% of the particle mass, were isolated from the hemolymph or oocytes of the tobacco hornworm, Manduca sexta. Examination of these subspecies by electron microscopy revealed distinctive morphologies. Adult high density lipophorin (HDLp-A) was found to be an approximately spherical particle with a diameter of 15 +/- 1 nm, while HDLp-Wanderer 1 (W1), was more rectangular in shape and had a distinct cleft extending into the particle at one end. In the case of HDLp-Wanderer 2 (W2) the cleft was deeper and wider than that in HDLp-W1. In egg very high density lipophorin (VHDLp-E) the cleft was increased in size to the extent that the particle had an overall crescent-like conformation. Circular dichroism spectroscopy of the three lipophorin subspecies that contain only apolipophorin I and II revealed that only minor differences in the global protein secondary structure occur as the particle lipid content is decreased. The VHDLp-E apolipoproteins are an exception in that, while having the same alpha-helix content as HDLp-W1 and HDLp-W2, they contain less beta-structure and correspondingly more random coil. Limited digestion of the apolipoprotein components of the lipophorin subspecies with trypsin revealed that as the lipid content of the particles decreases the susceptibility of the apolipoprotein to proteolytic degradation increases. Likewise, tryptophan fluorescence quenching experiments demonstrated that the relative exposure of lipophorin apolipoprotein tryptophan residues also increases as the particle lipid content decreases.(ABSTRACT TRUNCATED AT 250 WORDS)
从烟草天蛾（烟草天蛾）的血淋巴或卵母细胞中分离出主要昆虫脂蛋白——脂载蛋白的四个不同亚种，其总脂质含量占颗粒质量的20%至51%。通过电子显微镜检查这些亚种发现了独特的形态。发现成年高密度脂载蛋白（HDLp-A）是一个直径约为15±1纳米的近似球形颗粒，而HDLp-Wanderer 1（W1）形状更呈长方形，一端有一条明显的裂缝延伸到颗粒内部。对于HDLp-Wanderer 2（W2），其裂缝比HDLp-W1更深更宽。在卵中的极高密度脂载蛋白（VHDLp-E）中，裂缝尺寸增大到颗粒整体呈新月形的程度。对仅含有载脂蛋白I和II的三种脂载蛋白亚种进行圆二色光谱分析表明，随着颗粒脂质含量降低，整体蛋白质二级结构仅存在微小差异。VHDLp-E载脂蛋白是个例外，虽然其α-螺旋含量与HDLp-W1和HDLp-W2相同，但它们含有的β-结构较少，相应地随机卷曲较多。用胰蛋白酶对脂载蛋白亚种的载脂蛋白成分进行有限消化表明，随着颗粒脂质含量降低，载脂蛋白对蛋白水解降解的敏感性增加。同样，色氨酸荧光猝灭实验表明，随着颗粒脂质含量降低，脂载蛋白色氨酸残基的相对暴露程度也增加。（摘要截短于250字）
