Subcellular distribution of hepatic copper in beef cattle receiving high copper supplementation

Previous studies of intensively reared cattle in NW Spain have reported significantly higher copper (Cu) accumulation in the liver in Holstein-Friesian (HF) animals than in Galician Blonde (GB) or GBxHF crosses when receiving a diet supplemented at the maximum Cu concentrations allowed in the EU legislation (35 mg/kg). The present study aimed to evaluate whether this difference is due to the pattern of subcellular accumulation of Cu in the liver. For this purpose, liver samples from 10 GB, 9 HF and 10 GBxHF young bulls were analysed to determine the content of metallothionein (MT) and Cu and zinc (Zn) (in the liver (Cu-liver and Zn-liver) and bound to metallothionein (Cu-MT and Zn-MT)). The Cu distribution within the main subcellular compartments (nuclei, large granule, microsomes and cytosol) was also determined. Even though HF animals showed significantly higher (P < 0.05) Cu concentrations in the liver (161 ± 10 mg/kg wet weight) compared with GB (132 ± 8 mg/kg), no breed-related differences were observed for any of the parameters considered in this study. Overall, the pattern of hepatic subcellular accumulation was similar to that previously described in cattle: (i) MT concentrations were lower than in other animal species but strongly related to hepatic Zn; (ii) a low proportion of Cu (6.61%) was bound to MT but this was strongly and negatively related to the Cu:Zn ratio in the liver cell; and (iii) the highest proportion of Cu (57.3%) was found in the large granule (lysosome containing) fraction. All these results indicate a low capacity of cattle to excrete Cu by the bile resulting in a high Cu accumulation in the liver cell.