Studies on the efflux of heme from biological membranes

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.