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ABSTRACT
Three experiments were conducted by in vitro incubations in ruminai fluid to investigate the effects of pH and amounts of linoleic and linolenic acids on the extent of their biohydrogenation, the proportions of conjugated linoleic acid (CLA) and irons-CIS:! as intermediates, and the ratio irans-10:irans-ll intermediates. The effects of pH and amount of linoleic acid were investigated in kinetic studies, and effects of the amount of linolenic acid were studied with 6-h incubations. With identical initial amounts of linoleic acid, its disappearance declined when the mean pH during incubation was under 6.0 compared with a mean pH over 6.5, and when the amount of linolenic acid increased from 10 to 180 mg/160-ml flask, suggesting an inhibition of the isomerization step of the biohydrogenation. Low pH decreased the ratio of trans-10:trans11 intermediates. With initial amounts of linoleic acid increasing from 100 to 300 mg, the percentage of linoleic acid disappearance declined, but the amount that disappeared increased, without modification of the trans-lQ:trans-lI ratio, suggesting a maximal capacity of isomerization rather than an inhibition. Moreover, increasing initial linoleic acid resulted in high amounts of trans-Cl8:l and an increase of C18:0 that was a linear function of time, suggesting a maximal capacity for the second reduction step of biohydrogenation. High amounts of initial linolenic acid did not affect the amounts of CLA, trans-G18:1, or the ratio trans-W:trans-1I. Based on these experiments, a ruminal pH near neutrality with high amount of dietary linoleic acid should modulate the reactions of biohydrogenation in a way that supports CLA and trans11C18:1 in the rumen.
(Key words: biohydrogenation, conjugated linoleic acid, in vitro, polyunsaturated fatty acids)
Abbreviation key: BH = biohydrogenation, CLA = conjugated linoleic acid, FA = fatty acids, HpH = highpH buffer, LpH = low-pH buffer, PUFA = polyunsaturated fatty acids, t10 FA = trans-WC18:1 + trans-10, cis-12C18:2, t11FA = irans-HC18:1 + cis-9, trans11C18:2.
INTRODUCTION
Conjugated linoleic acid (CLA) groups positional and geometric isomers of linoleic acid (cis-9,cis12018:2, which will be referred as cis-C18:2) (Ha et al., 1989). Among them, cis-9,trans-HC18:2 and trans10,cis-12C18:2 are the most abundant in the human food (Chin et al., 1992), and have been shown to have specific dietetic properties: ciS-9,£rcms-11C18:2 inhibits carcinogenesis (Parodi, 1999) and trans-W,cis12C18:2 affects lipid metabolism (Park et al., 1999)....