THE MECHANISM OF ZINC UPTAKE BY CULTURED RAT-LIVER CELLS

1. The initial rate of Zn-65 uptake into cultured rat hepatocytes has been measured over a range of Zn2+ concentrations from 3 x 10(-10) M to 5 x 10(-6) M. Histidine and albumin were used to buffer Zn2+ ions at concentrations below 1 x 10(-6) M. 2. The results suggest there are two mechanisms for Zn2+ uptake; a high-affinity, saturable pathway, with a maximum velocity (V-max) of 20-30 pmol (mg protein)(-1) min(-1) and a Michaelis-Menten constant (K-m) of about 2 x 10(-9) M Zn2+ (with histidine), and a low-affinity, linear pathway, that only makes a significant contribution to Zn2+ uptake at Zn2+ concentrations above 1 x 10(-6) M. 3. Transport via the high-affinity pathway is dependent on the concentration of Zn2+ ions and not on the concentrations of Zn2+-ligand complexes, suggesting that Zn2+ is the transported species. 4. The affinity of the saturable pathway for Zn2+ is slightly lower in the presence of albumin, with a K-m of about 1.3 x 10(-8) M. The reason for this is uncertain.