Initial heme uptake from albumin by short-term cultured rat hepatocytes is mediated by a transport mechanism differing from that of other organic anions

Abstract

Although it is known that circulating heme accumulates in liver cells, the process by which heme enters hepatocytes is only partly understood. Hemopexin and a putative hemopexin receptor on hepatocyte membranes may mediate the uptake process. However, whether there are sufficient hemopexin receptors on rat hepatocytes to account for the bulk of heme entering cells is unknown. It is likely that heme may be transferred directly from albumin with the help of a plasma membrane heme transporter. To clarify the transport mechanism of heme into liver cells, we studied the uptake by short-term cultured rat hepatocytes of 55 Feheme incubated with rat serum albumin. In these cells, the initial uptake of 55 Fe-heme at 37ЊC was five-to eightfold higher than that at 4ЊC, linear for at least 5 minutes, and saturable. The K m of heme uptake was 0.95 ؎ 0.27 mol/L, and the V max was 0.12 ؎ 0.01 pmol/min/mg protein (n ‫؍‬ 3). Neither isosmotic substitution of sucrose for NaCl in the medium nor adenosine triphosphate (ATP) depletion, perturbations that are known to reduce uptake of bilirubin, sulfobromophthalein (BSP), and taurocholate, had any influence on 55 Fe-heme uptake. In addition, heme uptake was not reduced in the presence of a greater than 500-fold molar excess of BSP. These results indicate that hepatocytes take up heme by a process that is distinct from that of these other organic anions. (HEPATOLOGY 1998;28:150-155.)

Hemopexin: Structure, Function, and Regulation

“…Furthermore, cultured rat hepatocytes exhibit only about 2,000 HPX receptors per cell, which is not consistent with the magnitude of heme flux into hepatocytes. Finally, it has been shown that primary rat hepatocytes can uptake heme directly from albumin (Noyer et al, 1998), and it has been suggested that, after crossing the plasma membrane, heme could be bound by cytosolic heme-binding proteins such as fatty acid binding protein, glutathione S-transferase, mouse stress-inducible 23 kDa protein (MSP23), or the rat homologous HBP23 or even the recently cloned p22 HBP (Harvey and Beutler, 1982;Vincent and Muller-Eberhard, 1985;Ishii et al, 1993Ishii et al, , 1995Immenschuh et al, 1995a;Taketani et al, 1998a).…”

HeLp, a Heme Lipoprotein from the Hemolymph of the Cattle Tick,Boophilus microplus

“…However, Boophilus microplus is not able to synthesize its own heme; therefore, we suggest that an obligatory counterpart of tick dependence on heme from its host should be the development of efficient ways to absorb heme from the midgut and transport this molecule to tissues. In mammals, there are two proteins involved in extracellular heme transport: hemopexin and albumin (12). As HeLp is the main heme-protein in the hemolymph, it would be a candidate to act as a vehicle in transport of heme to tissues.…”

“…Due to its potential toxicity, heme is always found associated with proteins, such as hemopexin and albumin, and several antioxidant mechanisms have been developed to protect cells (7)(8)(9)(10)(11). As described in the literature, heme transport and recycling does not occur in eukaryotic cells where the heme delivered to cells is degraded by heme oxygenase (1,12).…”

Systemic regulation of Hephaestin and Ireg1 revealed in studies of genetic and nutritional iron deficiency

“…Heme and nonheme iron in the diet enter enterocytes by distinct paths but follow a convergent export route. The means by which heme is actively transported into intestinal enterocytes 3 or other mammalian cells 4 remains unknown, but once within enterocytes, heme oxygenase releases the iron from the heme. 5 Nonheme iron uptake has been better characterized.…”