Evans A J, Burley R W
J Biol Chem. 1987 Jan 15;262(2):501-4.
We report an example of the enzymic cleavage of an apoprotein B (apoB), the main apoprotein in the very low density lipoprotein (VLDL) of laying hens' blood, in a normal biological process, the formation of egg yolk. Plasma VLDL was labeled in vivo with 3H-amino acids, isolated by centrifuging, and injected into another laying hen. Yolk VLDL was isolated and its apoproteins were separated. ApoB was not detected in this lipoprotein. Most of the label originally in apoB was distributed among four smaller yolk apoproteins, apovitellenins III to VI, which are a large proportion of the apoproteins of VLDL in yolk. This distribution of 3H suggested that 80% of apoB was cleaved at three places. One yolk apoprotein, apovitellenin II, was not labeled, indicating that it did not originate from an apoprotein in plasma VLDL. The site for cleavage of apoB in the ovarian tissue has not been determined, but cleavage may occur during receptor-mediated endocytosis. The pattern of cleavage of apoB during transfer to yolk was not imitated by some known proteolytic enzymes.
我们报道了一个在正常生物过程——卵黄形成过程中，对产蛋母鸡血液极低密度脂蛋白（VLDL）中的主要载脂蛋白载脂蛋白B（apoB）进行酶促切割的实例。通过用³H-氨基酸在体内标记血浆VLDL，经离心分离后，将其注入另一只产蛋母鸡体内。分离出卵黄VLDL并分离其载脂蛋白。在这种脂蛋白中未检测到apoB。最初存在于apoB中的大部分标记分布在四种较小的卵黄载脂蛋白中，即卵黄磷蛋白III至VI，它们在卵黄VLDL的载脂蛋白中占很大比例。³H的这种分布表明，80%的apoB在三个位点被切割。一种卵黄载脂蛋白，即卵黄磷蛋白II，未被标记，这表明它并非源自血浆VLDL中的载脂蛋白。卵巢组织中apoB的切割位点尚未确定，但切割可能发生在受体介导的内吞作用过程中。一些已知的蛋白水解酶并未模拟apoB在转移至卵黄过程中的切割模式。
