Pleiotropic effect of LEC mutation: a rodent model of Wilson's disease

Abstract

Metabolic studies with 67Cu were undertaken to identify the site of the cellular defect in copper metabolism in the Long-Evans Cinnamon (LEC) rat. The apparent rate of copper uptake by LEC primary hepatocytes was increased [maximal velocity (Vmax) = 259 pmol.min-1.mg protein-1] compared with controls (Vmax = 161 pmol.min-1.mg protein-1); however, Michaelis-Menten constant (Km) values were comparable (11.8 and 12.7 microM, LEC and control, respectively). Efflux of copper from LEC and control hepatocytes was similar from 0 to 15 min, but was reduced from 15 to 60 min in the former. Although hepatic copper contents were markedly elevated in LEC rats compared with controls (658 +/- 199 vs. 21.5 +/- 6.6 micrograms/g dry wt), biliary copper concentration was reduced in LEC rats compared with controls (0.187 vs. 1.39 +/- 0.66 microgram/ml). Subcellular fractionation of LEC liver homogenates revealed approximately 75% of copper to be present in cytosol, with gradients of copper concentration from cytosol to either lysosome or microsomal subcellular fractions. LEC rat bile and hepatic microsome and lysosome fractions contained smaller fractions of 67Cu administered intravenously as cupric acetate compared with control rats. However, recovery of 67Cu in bile and in lysosomal subcellular fractions were similar for LEC and controls following administration of 67Cu-labeled asialoceruloplasmin, which is targeted to lysosomes. This discordance suggests a possible defect in the entry of copper into lysosomes but normal delivery of lysosomal copper to bile. Based on these findings, we conclude that the mutation in LEC rats alters copper transport at more than one cellular site.