Abstract:
38A Is acid strength a factor in the development of ruminal acidosis? P.M. May1 and J.B. Rowe Animal Science, University of New England, Armidale NSW 1 Current Address, School of Chemistr y, Murdoch University, Murdoch WA 6150 may@murdoch.edu.au It is sometimes claimed that differences in behaviour between organic acids involved in ruminal acidosis are due, at least in part, to their respective strengths as acids. For example, in their pioneering study Williams and Mackenzie (1965) note that 'Lactic acid is ... a stronger acid than the volatile fatty acids (VFA) (pKa's 3.86 and approximately 4.7 respectively)'. The protonation of the anions to form the corresponding (electrically neutral) acid species is also often invoked in mechanistic descriptions of how these molecules diffuse across lipid membranes. All these suggestions appear to overlook the very small extent to which any of these carboxylates can possibly exist in their protonated form under physiologically _ relevant conditions. The figure shows the protonation behaviour of each of the acids at 39 � C as calculated by the JESS (Joint Expert Speciation System) computer package (May and Murray 1991). The model was based on a comprehensive thermodynamic database, comprising over 1000 equilibrium constants from the chemical literature, critically assessed and transformed into a thermodynamically consistent set of mass balance equations. The ionic strength of the solution was fixed at 0.3 M and the redox potential, Eh, at 0 mV. The calculation was performed in terms of _log[H+] rather than pH since the latter is defined in a way that cannot be related exactly to the concentrations of chemical species in solution (Linder et al. 1984). It can be seen that over the interval 7.0> _log[H+] >5.0 only a small fraction of the volatile fatty acids ever become protonated. It follows that differences in physiological behaviour between the volatile fatty acids and lactic acid above, say, pH = 4.8 (corresponding approximately to _log[H+] = 5.0) are unlikely to be attributable to the differences in acid strength of the carboxylate compounds. Moreover, any manifestation of acidity difference between the acids would be much reduced by the pH buffering action of water under the significantly more reducing conditions which pertain in the rumen. For example, at Eh = _310 mV the calculated values of _log[H+] for 10mM solutions of acetic acid, lactic acid and hydrochloric acid at 39�C and I = 0.3 M are 4.4, 4.3 and 4.3 respectively compared to 3.2, 2.8 and 2.0 when Eh = 0 mV. These observations imply that it is far more likely that low pH associated with the presence of lactic acid in the rumen or hind gut is due to lack of lactic acid absorption from these gut compartments rather than the fact it is a stronger acid than the VFA. Linder, P.W., Torrington, R.G. and Williams, D.R. (1984). Analysis Using Glass Electrodes. Open University, Milton Keynes, UK. May, P.M. and Murray, K. (1991). JESS, A Joint Expert Speciation System -- I. Raison d'�tre. Talanta 38, 1409_1417. Williams, V.J. and Mackenzie, D.D.S. (1965). The absorption of lactic acid from the reticulo_rumen of the sheep. Australian Journal of Biological Science 18, 917_934. 100.0 Propanoic a cid 80.0 60.0 40.0 20.0 0.0 7 Buta noic ac id Acetic acid Lac tic ac id % 6.5 6 5.5 -log[H+] 5 4.5 4 Figure 1 Percentage protonation of various carboxylic acids. Recent Advances in Animal Nutrition in Australia, Volume 13 (2001)