ADENOSINE TRIPHOSPHATE-DEPENDENT COPPER TRANSPORT IN HUMAN LIVER

Background/Aim: The recent cloning and sequencing of the Wilson diseas e 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 membr anes (endoplasmic reticulum (ER), lysosomes) and/or in the bile canali cular membrane, The objective of this study was to establish character istics 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 canalicul ar 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 (Cu-64, 10 mu M) into cLPM vesicles and, to a lesser extent, into blLPM and XLPM was clearly stimulated when compared to control AMP values. Initial up take 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 s howed no relationship with marker enzyme activity of ER and lysosomes (glucose-6-phosphatase and acid-phosphatase, respectively). Leucine am inopeptidase activity, as a marker for the cLPM, significantly correla ted with ATP-dependent uptake rates measured in different membrane pre parations: r=0.70 (n=9, p<0.05). Estimated K-m and V-max values of ATP -dependent Cu uptake were 49.5 mu M and 36.9 nmol . min(-1). mg(-1) pr otein, respectively. Conclusion: This study provides biochemical evide nce for the presence of an ATP-dependent Cu transport system in human liver (cCOP), mainly localized at the canalicular domain of the hepato cytic plasma membrane.