Metal chelates of 2-hydroxy-4-methylthiobutanoic acid in animal feeding

The α-hydroxyacid 2-hydroxy-4-methylthiobutanoic acid (the so-called methionine hydroxy-analogue, MHA), largely used in animal nutrition as a source of methionine, forms stable metal chelates with divalent metals of formula [{CH3SCH2 CH2CH(OH)COO}2M] · nH2O. Protonation and iron(III) and copper(II) complex formation constants have been determined by potentiometry at 25 °C. Distribution diagrams show that no free Fe3+ cations are present in solution at pH > 2.5. ESI-MS (Electron-Spray Ionization Mass Spectrometry) investigations carried out both on iron and zinc complexes in solution have evidenced various species with different MHA/metal ratios. In vivo trials were carried out with rats. 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. Experiments in vitro with human intestinal Caco-2 cells indicated that the MHA/Fe chelate was taken up by the cells without any apparent toxic effect. The iron uptake was higher than that of iron nitrilotriacetate (Fe3+NTA), an effective chelate for delivering iron to milk diets. In conclusion, these data indicate that the use of MHA chelates could be a valuable tool to increase bioavailability of trace minerals and reduce the environmental impact of animal manure.