Liem H H, Noy N, Muller-Eberhard U
Division of Nutritional Sciences, Cornell University, Ithaca, NY 14853.
Biochim Biophys Acta. 1994 Sep 14;1194(2):264-70. doi: 10.1016/0005-2736(94)90308-5.
It is unknown how heme is distributed intracellularly from its site of synthesis in the mitochondria to other organelles. In previous work (Biochemistry 23, 3715, 1984) the transfer of heme from lipid bilayers to soluble proteins had been found to be independent of the recipient proteins' affinity for heme. Here, we investigated whether proteins are involved in the transfer of heme from biological membranes into aqueous media. We followed the release of 14C-labeled heme, from mitochondria preloaded with the heme, to BSA and found that only about 28%, of the heme was extracted on the first wash. After the third wash 35-50% of the heme that had been partitioned into the membranes was extracted. Fourth and fifth washes with BSA or a cytosolic heme-binding protein (HBP, also known as liver fatty acid binding protein) removed only insignificant amounts of 14C-labeled heme. Similarly, a large portion of the preloaded 14C-labeled heme could not be extracted from a variety of isolated membranes (inner and outer mitochondrial membranes, plasma membranes of liver cells, kidney cortex cells and erythrocyte membranes). By contrast, essentially all [14C]palmitate preloaded in biological membranes and all 14C-labeled heme preloaded in synthetic membranes was released to albumin (Biochemistry 23, 3715, 1984). These observations suggest that, in general, heme associates with membrane components which can be distinguished into two compartments. One compartment releases its heme spontaneously, while another compartment binds heme so tightly that a specific process has to be evoked for its release.
目前尚不清楚血红素是如何在线粒体内从其合成位点转运到其他细胞器的。在之前的研究中（《生物化学》23卷，3715页，1984年），已发现血红素从脂质双层转移到可溶性蛋白质的过程与受体蛋白对血红素的亲和力无关。在此，我们研究了蛋白质是否参与血红素从生物膜转移到水相介质的过程。我们追踪了预先加载有血红素的线粒体中14C标记的血红素释放到牛血清白蛋白（BSA）中的情况，发现第一次洗涤时仅提取了约28%的血红素。第三次洗涤后，已分配到膜中的血红素的35 - 50%被提取出来。用BSA或一种胞质血红素结合蛋白（HBP，也称为肝脏脂肪酸结合蛋白）进行第四次和第五次洗涤，仅去除了少量的14C标记的血红素。同样，大量预先加载的14C标记的血红素无法从多种分离的膜（线粒体内外膜、肝细胞、肾皮质细胞的质膜和红细胞膜）中提取出来。相比之下，基本上所有预先加载在生物膜中的[14C]棕榈酸盐和所有预先加载在合成膜中的14C标记的血红素都释放到了白蛋白中（《生物化学》23卷，3715页，1984年）。这些观察结果表明，一般来说，血红素与膜成分结合，这些膜成分可分为两个部分。一个部分会自发释放其血红素，而另一个部分则紧密结合血红素，以至于必须引发特定过程才能使其释放。
