Dose dependence of zinc and manganese absorption in man

Intestinal absorption and excretion are the main regulatory mechanisms for maintenance of body homeostasis of most trace elements.In spite of considerable research on trace element metabolism in man and experimental animals, the knowledge about the mechanisms involved in these processes is still limited.The characteristics of trace element metabolism, such as low fractional absorption, the presence of the elements in intestinal juices and a rapid plasma turn-over, complicate specific studies of intestinal trace element absorption and excretion in the intact animal or man.Several in vivo and in vitro experimental models have, therefore, been developed, e.g.everted intestinal sacs, in situ loops, brush-border-membrane vesicle preparations and intestinal perfusion techniques, to study the kinetics of trace element absorption, usually administering aqueous solutions of the element at different concentrations or with the addition of ligands.However, the chemical environment at the site for intestinal absorption after ingestion of food is much more complex with the presence of partly digested food components, amino acids, organic acids, competing ions, etc.Therefore, the results from experimental models have to be judged with caution. ZINC ABSORPTIONSite of Z n absorption.Zn seems to be absorbed throughout the entire small intestine.Intestinal perfusion studies of isolated intestinal segment in rats, using a perfusate containing 10 p,g Zdml (0.15 mM), have suggested that the capacity of Zn absorption is highest in the ileum (Antonson et al. 1979).Davies (1980) used an intralumen injection of 10 p,g Zn in 0.1 ml water labelled with 65Zn to rats and showed that the duodenum contributed 60%, the ileum 30% and the jejunum 10% to the overall absorption of Zn.Negligible absorption occurred from the caecum and colon.Using a triple-lumen technique in humans and an electrolyte solution containing 0.1 mM-ZinC acetate for the perfusion, Lee et al. (1989) observed the highest rate of absorption in the jejunum compared with the duodenum and ileum.As most of the hypotheses for Zn absorption mechanisms are based on results from studies of experimental animals, it would be important to find out whether the observed differences reflect true species differences or can be ascribed to differences in experimental design.Z n absorption mechanisms.Experimental models indicate that Zn absorption is a carrier-mediated process and that uptake at the brush-border membrane is the ratelimiting step.Menard & Cousins (1983) used brush-border-membrane vesicles from rat intestine to study the kinetics of Zn absorption and concluded that Zn uptake was saturable and occurring by a carrier-mediated process, that did not seem to be energy dependent at 0.2 mM.At 1 mM it was non-saturable indicating a passive diffusion process.The results suggest a Michaelis constant (Km) of 0.38 mM and a maximum uptake velocity (Jmm) of 5-4 nmol/min per mg protein.