Liu H, Ryan R O
Department of Biochemistry, University of Alberta, Edmonton, Canada.
Biochim Biophys Acta. 1991 Aug 20;1085(1):112-8. doi: 10.1016/0005-2760(91)90238-d.
Oocyte development in Manduca sexta involves the deposition of large amounts of lipoprotein-derived lipid. One source of this lipid is hemolymph high-density lipophorin-adult (HDLp-A) which, upon entry into the oocyte, is transformed into a lipid and apolipophorin III deficient product particle, egg very-high- density lipophorin (VHDLp-E; density = 1.24 g/ml; Kawooya et al. (1988) J. Biol. Chem. 263, 8740-8747). An in vitro model of this transformation has been established using human low-density lipoprotein (LDL) as acceptor of HDLp-A associated lipid in a reaction catalyzed by isolated M. sexta hemolymph lipid transfer particle (LTP). Facilitated vectorial net transfer of lipid from HDLp-A to LDL resulted in formation of a very-high-density lipophorin (VHDLp) product with a density and apolipoprotein content similar to that of VHDLp-E. Lipid was found to comprise 25% of the VHDLp particle mass, whereas over 50% of HDLp-A mass is lipid. Based on these observations it was hypothesized that a lipid transfer factor may be present in M. sexta oocytes and function in the transformation of HDLp-A to VHDLp-E in vivo. Transfer activity was present in the buffer soluble fraction of oocyte homogenates and purification of the active material revealed a catalyst with electrophoretic and immunological properties identical to hemolymph LTP. Incubation of 125I-HDLp-A with an M. sexta oocyte homogenate resulted in transformation of the radiolabeled lipoprotein to a density corresponding to that of VHDLp-E. When the incubation media was preincubated with anti-LTP IgG this conversion was inhibited to a large extent. Inhibition was relieved, however, by addition of exogenous LTP. The results provide the first demonstration of M. sexta LTP in a tissue other than hemolymph and support the concept that LTP-catalyzed lipid transfer plays an integral role in the conversion of HDLp-A to VHDLp-E in vivo.
烟草天蛾的卵母细胞发育涉及大量脂蛋白衍生脂质的沉积。这种脂质的一个来源是血淋巴高密度脂蛋白成虫型（HDLp-A），它进入卵母细胞后会转化为一种脂质和脱辅基脂蛋白III缺陷型产物颗粒，即卵极高密度脂蛋白（VHDLp-E；密度 = 1.24 g/ml；Kawooya等人，(1988)《生物化学杂志》263, 8740 - 8747）。利用人低密度脂蛋白（LDL）作为HDLp-A相关脂质的受体，在分离的烟草天蛾血淋巴脂质转运颗粒（LTP）催化的反应中，建立了这种转化的体外模型。脂质从HDLp-A到LDL的促进性向量净转移导致形成了一种极高密度脂蛋白（VHDLp）产物，其密度和载脂蛋白含量与VHDLp-E相似。发现脂质占VHDLp颗粒质量的25%，而HDLp-A质量的50%以上是脂质。基于这些观察结果，推测烟草天蛾卵母细胞中可能存在一种脂质转移因子，在体内HDLp-A向VHDLp-E的转化中起作用。转移活性存在于卵母细胞匀浆的缓冲液可溶部分，活性物质的纯化揭示了一种具有与血淋巴LTP相同电泳和免疫特性的催化剂。用125I-HDLp-A与烟草天蛾卵母细胞匀浆孵育导致放射性标记的脂蛋白转化为与VHDLp-E相对应的密度。当孵育介质与抗LTP IgG预孵育时，这种转化在很大程度上受到抑制。然而，通过添加外源性LTP可以解除抑制。这些结果首次证明了烟草天蛾LTP在血淋巴以外的组织中的存在，并支持了LTP催化的脂质转移在体内HDLp-A向VHDLp-E转化中起不可或缺作用的概念。
