The accumulation of copper by rat liver slices

The uptake of divalent manganese ion by mature normal human red blood cells

At physiological pH and concentrations of Mn⁺⁺ in excess of 5 x 10^-4M, study of the Mn⁺⁺ ion movement into human red cells is complicated by physicochemical alterations of the ion itself. At concentrations below 5 x 10^x4M, the rate of uptake bears a linear relationship to the Mn⁺⁺ concentration. The permeability constant for inward movement of Mn⁺⁺ is 2.87 ± 0.13 (S.E.) x 10^-9 cm./sec. The rate is not influenced by the addition of metabolic substrates such as glucose or adenosine or the metabolic inhibitors iodoacetate or fluoride. Co⁺⁺, Ca⁺⁺, and Mg⁺⁺ do not appear to compete with Mn⁺⁺ for entry, but at high concentrations relative to Mn⁺⁺, they reduce the rate of entry. Ca⁺⁺ is far more effective than Co⁺⁺ or Mg⁺⁺ in this regard. The permeability constant for outward Mn⁺⁺ movement is 1.38 ± 0.21 (S.E.) x 10^-9 cm./sec., about half of that for entry. This slower rate of outward movement is consistent with the finding that 40 to 60 per cent of the Mn⁺⁺ taken up by the red cells is non-ultrafilterable. Less than 5 to 10 per cent of the Mn⁺⁺ appears to be bound to the stroma. It is concluded that entry and exit of Mn⁺⁺ is a process of passive diffusion involving no carriers, transport, or metabolic linkage.

Mechanisms of metal transport across liver cell plasma membranes

The liver's pivotal role in the homeostasis of essential trace metals and detoxification of exogenous metals is attributed to its ability to efficiently extract metals from plasma, metabolize, store, and redistribute them in various forms either into bile or back into the bloodstream. Bidirectional transport across the sinusoidal plasma membrane allows the liver to control plasma concentrations and therefore availability to other tissues. In contrast, transport across the canalicular membrane is largely, but not exclusively, unidirectional and is a major excretory pathway. Although each metal has relatively distinct hepatic transport characteristics, some generalizations can be made. First, movement of metals from plasma to bile follows primarily a transcellular route. The roles of the paracellular pathway and of ductular secretion appear minimal. Second, intracellular binding proteins and in particular metallothionein play only indirect roles in transmembrane flux. The amounts of metallothionein normally secreted into plasma and bile are quite small and cannot account for total metal efflux. Third, metals traverse liver cell plasma membranes largely by facilitated diffusion, and by fluid-phase, adsorptive, and receptor-mediated endocytosis/exocytosis. There is currently no evidence for primary active transport. Because of the high rate of hepatocellular membrane turnover, metal transport via endocytic vesicles probably makes a larger contribution than previously recognized. Finally, there is significant overlap in substrate specificity on the putative membrane carriers for the essential trace metals. For example, zinc and copper share many transport characteristics and apparently compete for at least one common transport pathway. Similarly, canalicular transport of five of the metals discussed in this overview (Cu, Zn, Cd, Hg, and Pb) is linked to biliary GSH excretion. These metals may be transported as GSH complexes by the canalicular glutathione transport system(s). Unfortunately, none of the putative membrane carrier proteins have been studied at the subcellular or molecular level. Our knowledge of their biochemical properties is rudimentary and rests almost entirely on indirect evidence obtained in vivo or in intact cell systems. The challenge for the future is to isolate and characterize these putative metal carriers, and to determine how they are functionally regulated.

Uptake of 67copper complexed to 3H-histidine by brain hypothalamic slices: evidence that dissociation of the complex is not the only factor determining 67copper uptake

It was previously shown that complexation of 67Cu with His facilitates 67Cu uptake by hypothalamic slices and that His, in a concentration that is 1000-fold greater than Cu(His)2, inhibits 67Cu uptake (D. E. Hartter and A. Barnea, J. Biol. Chem. 263, 799-805 (1988)). We addressed the question: Does dissociation of the Cu(His)2 complex occur during the process of Cu2+ uptake and if so, is dissociation the only factor determining uptake? Rat hypothalamic slices were incubated with 67Cu(3H-His)2 and the kinetic profiles of 67Cu and 3H-His uptake were evaluated. 67Cu uptake was linear for up to 60 min, Vo vs S [0.1-160 microM Cu(His)2] was sigmoidal, Lineweaver-Burk plot was non-linear, Scatchard plot was bell-shaped, and Hill plot had multiple slopes. In contrast, 3H-His uptake was linear for up to 30 min, Vo vs S was biphasic, Lineweaver-Burk plot was linear, Scatchard plot was biphasic, and Hill plot had a single slope. Keeping [67Cu] constant and increasing [3H-His] resulted in a dose-dependent inhibition of 67Cu uptake which was not accompanied by an inhibition of 3H-His uptake. Substituting His in the complex with Phe or Lys resulted in a marked shift to the right in Vo vs S for 67Cu uptake and at S less than 40 microM, only His facilitated 67Cu uptake relative to ionic 67Cu2+. However, Vo vs S for 3H-His, 3H-Phe, and 3H-Lys uptake were superimposeable, indicating comparable dissociation of the complexes. In summary, we demonstrate that, although complexation of Cu2+ is essential for 67Cu uptake by hypothalamic tissue, 67Cu and 3H-His are taken up by distinct processes, which implies dissociation of the complex at the level of the membrane. Moreover, even though dissociation occurs, it is not the only factor that determines Cu2+ uptake by the hypothalamic tissue. It is suggested that the physicochemical properties of the Cu complex is an important factor determining Cu uptake by brain tissue.

High-affinity uptake of 67Cu into a veratridine-releasable pool in brain tissue

We have previously characterized two saturable, ligand-dependent processes for 67Cu uptake by hypothalamic slices: a high- and low-affinity process (22). In this study, we wished to ascertain if veratridine, a secretagogue that mimics a physiological release process, stimulates the release of newly taken up 67Cu and whether uptake of 67Cu into the releasable pool of copper is dependent on the process of 67Cu uptake. Hypothalamic or caudate slices from male rats were loaded for 30 min with 67Cu complexed to histidine (His) under conditions favoring high- or low-affinity uptake. First, we assessed the stability of the newly taken up 67Cu and found that, regardless of the mode of 67Cu entry into the tissue, greater than or equal to 85% of the 67Cu is retained in tissues incubated for 3 h in 67Cu-free buffer. Moreover, the 67Cu taken up by the high-affinity process was not displaced by 15-fold molar excess of nonradiolabeled Cu2+, histidine, albumin, or Zn2+, and only 20-30% of the 67Cu taken up by the low-affinity process was displaced by 10-fold excess Cu2+ or albumin. Next, we assessed veratridine stimulation of 67Cu release and found that 67Cu release occurred only from tissues loaded with the high- but not with the low-affinity process. This effect of veratridine was calcium dependent and was blocked by Tetrodotoxin, a specific blocker of the voltage-sensitive Na+ channel. In addition, we confirmed our earlier observation that a depolarizing concentration of K+ stimulates 67Cu release.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake of copper by mouse hepatocytes

This study has investigated the uptake of copper by mouse hepatocytes. The cells gave similar results whether they were used right after isolation or maintained overnight on collagen-coated dishes. Uptake from cells in suspension followed two phases: an initial rapid binding followed by a linear uptake phase. The two phases were not so easily distinguishable in cells grown in culture where uptake was linear over the first hour. The uptake showed saturation but may not have followed simple kinetics. Histidine stimulated uptake in a concentration-dependent manner, as did some other amino acids, but copper had very little effect on histidine uptake. The process was not dependent on intracellular adenosine triphosphate (ATP), since inhibitors that substantially reduced ATP levels inside the cell did not alter copper uptake. The inhibitors, however, blocked histidine uptake to varying degrees, suggesting that copper and histidine are taken up by different pathways. The uptake was reduced markedly by N-ethyl maleimide, and preincubation of the cells with "Pronase" resulted in a decrease of uptake. A model for the uptake of copper by hepatocytes that incorporates the data presented in this paper with that produced by earlier workers is suggested.

Transport and intracellular distribution of copper in a human hepatoblastoma cell line, HepG2

The uptake of radiocopper by HepG2 cells is a saturable, temperature-dependent and cellular energy-independent process with a Vmax of 7.1 +/- 0.2 pmoles min-1 mg protein-1 and an estimated Km of 3.3 +/- 0.5 microM. The rate of copper uptake is reduced at an equimolar concentration of albumin and is unaffected by zinc at a 10-fold molar excess. Approximately 70% of the newly incorporated radiocopper binds to membranes and organelles, while 30% is recovered in the cytosol. The soluble fraction can be resolved into two copper-binding protein peaks. Incubation of HepG2 with nonisotopic copper results in displacement of radiocopper associated with the proteins contained in the lower molecular weight peak. Exposure of the cells to cycloheximide inhibits the incorporation of the isotope into this fraction.