Mobilization of copper(II) from plasma components and mechanism of hepatic copper transport

The effects of plasma Cu(II) ligands on the kinetics of Cu(II) transport by rat liver parenchymal cells were determined to examine how Cu(II) is mobilized from plasma and transported into liver cells. Albumin markedly inhibited Cu(II) uptake at Cu(II)-to-albumin molar ratios of 3:1 or less. Kinetic analyses showed that albumin inhibits Cu(II) uptake by reducing the concentration of free Cu(II) in solution. Under conditions of excess albumin to Cu(II), histidine facilitated albumin-inhibited uptake of Cu(II). Threonine, glutamine, and most other amino acids were without effect. Moreover, the facilitation efffect of a low-molecular-weight plasma fraction (≤5,000) was largely accounted for by its histidine concentration. The tripeptide Gly-His-Lys also inhibited Cu(II) uptake into hepatocytes by the same mechanism as albumin. The inhibitory effects of albumin and Gly-His-Lys were additive with or without histidine. The active species in the Cu(II), albumin, and histamine mixtures was shown to be the His₂Cu(II) complex. V(max) for this complex was identical to the V(max) for free Cu(II), but the K(m) was slightly higher [15 μM vs. 11 μM for free Cu(II)]. Concurrent determinations of [³H]-histidine and ⁶⁴cu(II) uptake showed that histidine was not transported with Cu(II) from His.Cu(II) or His₂Cu(II) complexes. The data are consistent with histidine mobilizing Cu(II) from albumin by competing for Cu(II), interaction of the His₂Cu(II) complex with the putative hepatic copper transport protein, and transport of copper as free ionic copper.