A proposed mechanism of zinc absorption in the rat.

Studies were conducted at the cellular level in an attempt to describe the processes involved in zinc absorption from the intestine. A low-molecular-weight zinc-binding ligand was identified in the pancreas of rats and pancreatic secretions from a dog. The whole-body absorption of 65Zn in rats in which the common bile duct was ligated was significantly less than the absorption of 65Zn in rats in which the hepatic bile duct was ligated. The uptake of 65Zn by epithelial cells from everted intestinal segments was markedly increased in the presence of the zinc-binding ligand fraction from pancreatic secretions. Following in vivo labeling, 30% of the 65Zn in the epithelial cell was associated with the partially purified basolateral plasma membrane. When labeled basolateral plasma membranes were incubated in a medium that contained zinc-free albumin, approximately 96% of the 65Zn was transferred to the medium while less than 30% of the isotope was released to media that contained either no albumin or a 3:1 zinc: albumin complex. In rats fed a zinc-deficient diet, 65Zn absorption was inversely proportional to the serum zinc concentration, and both zinc and copper injections produced a marked decrease in 65Zn absorption. These results suggest that zinc absorption consists of interactions among a low-molecular-weight ligand, recpetor sites on the basolateral membrane, and metal-free albumin.