Characteristics of uptake systems for copper delivered to cells as copper‐histidine (Cu‐his) and on plasma proteins

In the blood plasma, Cu is bound to components larger than 10 kDa. Albumin and transcuprein (a macroglobulin) appear to deliver it directly to cells, but uptake has mostly been measured with Cu salts or Cu‐his. To determine whether different plasma proteins and Cu‐his deliver Cu to the same uptake systems and/or to Cu transporter 1 (Ctr1), uptake studies and kinetics were carried out with 64 Cu, with/without Ctr1 knockdown, and with or without Ag(I). siRNA knocked down expression of CTR1 mRNA 60–85% in human mammary epithelial and hepatic cell models respectively, but this had no effect on uptake of 1 μM Cu attached to pure albumin or alpha‐2‐macroglobulin. Mouse embryonic fibroblasts that did and did not express Ctr1 (kindly provided by Dennis Thiele) showed robust uptake of Cu from human and mouse albumin, with saturation kinetics, and Km values of about 3 and 11 μM (for Cu) respectively. Uptake from Cu(I)‐his had a higher Vmax but also a much higher Km. Uptake of Cu(II) from histidine was 10‐fold less rapid. Ag(I) had differential effects on uptake from albumin and histidine; and uptake was only a little lower in the fibroblasts without Ctr1. We conclude that plasma proteins deliver Cu to cells with greater efficiency than histidine at physiological concentrations, and confirm that Ctr1 is not the only means by which Cu is taken up by cells. Supported by U.S. Public Health Service Grant RO1 HD 46949.