Adenosine triphosphate-dependent copper transport in human liver

Abstract

Background/Aim: The recent cloning and sequencing of the Wilson disease gene indicates that hepatic copper (Cu) transport is mediated by a P-type ATPase. The location of this Cu-transporting protein within the hepatocyte is not known; in view of its proposed function and current concepts of hepatic Cu transport, it may reside in intracellular membranes (endoplasmic reticulum (ER), lysosomes) and/or in the bile canalicular membrane. The objective of this study was to establish characteristics and localization of ATP-dependent Cu transport in human liver. Methods: We have investigated Cu transport in vesicles of human liver plasma membranes showing a gradual increase in enrichment of canalicular domain markers: i.e. basolateral liver plasma membranes (blLPM), a mixed population of basolateral and canalicular (XLPM) and canalicular liver plasma membranes (cLPM). Results: In the presence of ATP (4 mM) and an ATP-regenerating system, uptake of radiolabeled Cu ( 64Cu, 10 μM) into cLPM vesicles and, to a lesser extent, into blLPM and XLPM was clearly stimulated when compared to control AMP values. Initial uptake rates of ATP-dependent Cu transport were 5.6, 7.8 and 13.7 nmol·min −1·mg −1 protein for blLPM, XLPM and cLPM, respectively, and showed no relationship with marker enzyme activity of ER and lysosomes (glucose-6-phosphatase and acid-phosphatase, respectively). Leucine aminopeptidase activity, as a marker for the cLPM, significantly correlated with ATP-dependent uptake rates measured in different membrane preparations: r=0.70 ( n=9, p<0.05). Estimated K m and V max values of ATP-dependent Cu uptake were 49.5 μM and 36.9 nmol·min −1·mg −1 protein, respectively. Conclusion: This study provides biochemical evidence for the presence of an ATP-dependent Cu transport system in human liver (cCOP), mainly localized at the canalicular domain of the hepatocytic plasma membrane.