Metal chelates of 2-hydroxy-4-methylthiobutanoic acid in animal feeding: characterization, in vitro and in vivo investigations.

Metal chelates of 2-hydroxy-4-methylthiobutanoic acid in animal feeding: characterization, in vitro and in vivo investigations Giacomo Biagi,a Enrico Cinti,b Simonetta Ferruzza,c Attilio L. Mordenti,a Giovanni Predieri,d Matteo Tegonid aDIMORFIPA, University of Bologna, Via Tolara di Sopra 50, Ozzano Emilia, Italy; bAgristudio S.r.l., Via Gramsci 56, Reggio Emilia, Italy; cINRAN, Nutrition National Institute, Via Ardeatina 546, Roma; aGIAF Chemistry Department, University of Parma, Parco delle Scienze, Parma, Italy Interest in using alternative mineral sources, particularly those chelated with proteins or aminoacids, has recently increased due to their reported higher availability compared to conventional (inorganic) sources [1]. For example, it has been found that ruminants respond (increased growth, milk production etc.) to certain trace mineral complexes or chelates [2] and that aminoacid chelates show a higher availability than the inorganic compounds when fed to rainbow trouts, even in presence of phosphates and phytates [3]. However, there are still contentions both regarding improved bio-availability and integrity of metal chelates at the low pH of the first digestive tract. This important drawback can be overcome by using a chelating ligand able to improve the stability of chelates at low pH values. Preliminary results of our investigations [4] indicate that 2-hydroxy-4-methylthiobutanoic acid (MHA; the so-called methionine hydroxy-analogue), an alpha-hydroxyacid largely used in animal nutrition as a source of methionine has the requested features. It forms bis-chelate complexes of formula [{CH3SCH2CH2CH(OH)COO}2M].nH2O (M = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Cu2+ or Zn2+). Potentiometric investigations for the copper(II), zinc(II) and iron(III) MHA chelates in solution show that these species, differently from AA chelates, are rather stable even at low pH values. In the case of iron, the distribution diagram of the system Fe3+/MHA does not show any trace of free iron cations at pH > 2.5. Furthermore, in order to gain insight about biavailability of MHA chelates, in vitro and in vivo investigations were performed. In the in vitro studies, human intestinal Caco-2 cells were exposed to Fe3+/MHA chelate solutions. The iron/MHA chelate did not alter the permeability of Caco-2 tight junctions and was taken up to a larger extent than the reference iron chelated with nitrilotriacetic acid. In vivo investigations were carried out in the rat. After receiving a zinc-deficient diet for 3 weeks, animals were fed the same diet added with zinc sulfate or zinc/MHA chelate; the zinc content of faeces was higher (+ 45%; P < 0.05) in sulfate fed rats, whereas zinc retention was higher (+ 61%; P < 0.05) in the Zn/MHA diet. References [1] Ashmead, H.D., 1992, The Roles of AA Chelates in Animal Nutrition, Moyes Publ., Park Ridge, NJ. [2] Spears, J.W., 1996, Anim. Feed Sci. Technol., 58, 151-163. [3] Satoh, S. et al., 2003, Aquacult., 225, 431-444. [4] Predieri, G. et al., 2003, J. Inorg. Biochem., 95, 221-224.