Absorption of copper and copper-histidine complexes across the apical surface of freshwater rainbow trout intestine

Bioavailability is integral in mediating the delicate balance between nutritive and potentially toxic levels of copper in fish diets. Brush-border membrane vesicles isolated from freshwater rainbow trout intestine were used to characterise apical copper absorption, and to examine the influence of the amino acid histidine on this process. In the absence of histidine, a low affinity, high capacity copper uptake mechanism was described. However, when expressed as a function of ionic copper (Cu²⁺), absorption was linear, rather than saturable, suggesting that the saturable curve was an artifact of copper speciation. Conversely, in the presence of l-histidine (780 μM) saturable uptake was characterised. The uptake capacity discerned (J _max of 354 ± 81 nmol mg protein^-1 min^-1) in the presence of histidine indicated a significantly reduced capacity for copper transport than that in the absence of histidine. To determine if copper uptake was achievable through putative histidine uptake pathways, copper and histidine were incubated in the presence of tenfold greater concentrations of amino acids proposed to block histidine transporters. Accounting for changes in copper speciation, significant inhibition of uptake by glycine and lysine were noted at copper levels of 699 and 1,028 μM. These results suggest that copper-histidine complexes may be transportable via specific amino acid-transporters in the brush-border membrane.