Letter to the Editor: Author responds to comments on zinc, copper absorptionLetter to the Editor: Author responds to comments on zinc, copper absorption
August 14, 2015
I AM writing in response to comments made by Drs. Terry Ward and Mike Socha (Feedstuffs, July 20) regarding my article in Feedstuffs titled "Advances in trace mineral absorption, homeostasis" (Feedstuffs, May 4).
As a professor at a major land-grant university, I have always attempted to conduct and report research results as well as review articles in an objective and unbiased manner.
I retired from North Carolina State University in 2012 after 32 years and currently work part time as an animal nutrition consultant. I have conducted research for, and acted as an independent consultant to, many of the companies commercially involved in mineral nutrition both as consumers of mineral products and as suppliers. Among my current clients are Kemin and Micronutrients.
An article I published 26 years ago (Spears, 1989) was one of the first peer-reviewed journal articles published on zinc methionine (ZnMet). In this paper, I used verbs such as could, suggest, may and appear to describe potential differences in metabolism between ZnMet and zinc oxide (ZnO).
My hypothesis of ZnMet and ZnO being metabolized differently after absorption was based on: (1) a tendency for lower urinary zinc excretion in lambs fed ZnMet compared to ZnO and (2) a slower rate of decline in plasma zinc in lambs given a pharmacological dose of ZnMet compared to ZnO.
The decrease in urinary zinc excretion was not significant in either of my two trials, and a later study indicated similar urinary excretion of zinc in steers supplemented with ZnMet and zinc sulfate (Nockels et al., 1993; J. Anim. Sci. 71:2539-2545).
As discussed within my Feedstuffs article, when zinc is supplemented well above the animal's requirement, passive absorption becomes the major pathway for zinc absorption. The slower rate of decline in plasma zinc in lambs administered ZnMet may have been due to the low solubility of ZnO negatively affecting passive absorption of zinc from ZnO under these conditions.
Literature cited in the Feedstuffs article does not refute my research but, rather, refutes my hypothesis that ZnMet may be absorbed and transported in the plasma intact. Hypotheses are intended to stimulate additional research to either support or reject the hypothesis.
Three of the papers cited by Ward and Socha in support of the absorption of zinc and copper amino acid complexes intact were conducted in fish (invertebrate) or with brush-border membranes prepared from rainbow trout intestines, not in mammals.
Ward and Socha also referred to a paper by Gao et al. that examined the absorption of copper by Caco-2 cells (a human colon carcinoma cell line). In most of the experiments reported in this paper, Caco-2 cells were exposed to media containing 97.5 micrograms of copper per milliliter. This would equate to a copper concentration in the media of 1,535 micromol, which greatly exceeds physiological concentrations found in the small intestine. This concentration of copper would likely cause toxicosis in animals.
Previous studies evaluating the absorption of copper by Caco-2 cells have used concentrations of copper in the media ranging from 0.2 to 94.0 micromol (Biochim. Biophy. Acta 1474:169-176, 2000; Am. J. Physiol. — Gastrointest. Liver Physiol. 284:G739-G747, 2003; J. Nutr. Biochem. 15:155-162, 2004). In pigs supplemented with 225 parts per million of copper, soluble copper concentrations in the duodenum and proximal jejunum were less than 6 micrograms/mL of digesta (J. Anim. Sci. 93:2948-2955, 2015).
Uptake of copper by the physiologically important copper transporter Ctr1 is saturable (J. Biol. Chem. 277:4380-4387, 2002). At the concentrations of copper used in the experiments by Gao et al., I agree that mechanisms (likely passive diffusion) other than Ctr1 would be involved in copper uptake by Caco-2 cells.
Furthermore, in the study where the Ctr1 inhibitor carboplatin was used, only absorption of copper from copper methionine complex was examined. Since inorganic copper was not evaluated, the results do not demonstrate that copper methionine complex is absorbed differently from inorganic copper, even at high concentrations.
The location of Ctr1 in the cell membrane of Caco-2 cells also appears to be different from the location found in pigs, rats and mice (J. Biol. Chem. 285:32385-32392, 2010). In Caco-2 cells, Ctr1 appears to reside on the basolateral membrane, while in animals, Ctr1 is on the apical membrane of intestinal epithelial cells.
I appreciate the opportunity to respond to the comments made by Ward and Socha.
Jerry W. Spears
Professor Emeritus of animal science and nutrition
North Carolina State University
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