The inhibitory effect of conjugated linoleic acid (CLA) on N-6 polyunsaturated fatty acid metabolism in a transformed yeast model.

摘要

Conjugated linoleic acid (CLA), a mixture of positional and geometric isomers derived from linoleic acid (LA; Delta9,12--18:2), has been shown to modulate immune function and inhibit certain types of cancer in cell culture and animal models. It has been postulated that CLA exerts its beneficial biological effect through suppression of production of n-6 polyunsaturated fatty acids (PUFA), especially arachidonic acid (AA; Delta5, 8,11,14--20:4) and consequently, pro-inflammatory eicosanoids. Since in the PUFA metabolic pathway, Delta6-desaturase, elongase and Delta5-desaturase are directly responsible for synthesis of AA, the hypothesis of this study is that CLA reduces the production of AA by inhibiting the activities of these enzymes. Because the mammalian microsomal system is complex and the purified enzymes are not available, a simple model for testing the hypothesis was needed for this study. Recently, different genes involved in PUFA metabolism have been identified and successfully expressed in baker's yeast, Sacchoromyces cerevisiae. Through the transformed yeast, the activity of Delta6-desaturase, elongase, or Delta5-desaturase could be investigated step-by-step. The objective of this study was to determine whether CLA isomers can affect the Delta6-desaturation which converts LA to gamma-linolenic acid (GLA; Delta6,9,12--18:3); elongation of LA to EDA (Delta11,14--20:2), elongation of GLA to dihomo-gamma-linolenic acid (DGLA; Delta8,11,14--20:3), and Delta5-desaturation of DGLA to AA. Results from this study showed that CLA clearly modulated n-6 PUFA metabolism by inhibiting the two rate-limiting steps of the n-6 PUFA metabolic pathway, Delta6-desaturation of LA and Delta5-desaturation of DGLA. CLA could also inhibit the alternate pathway, i.e. elongation of LA, but not elongation of GLA. The inhibitory effect of CLA on these enzymes is likely due to the competition between substrates and CLA for the enzymes. Among the four CLA isomers studied, c9,t11- and t10,c12-CLA isomers could be elongated to form conjugated EDA (CEDA) isomers by the elongase. These two CLA isomers and the c9,c11-CLA could also be metabolized to form Delta5-desaturation products by the Delta5-desaturase. Since these two CLA isomers also exerted the greatest inhibition, it is likely that these two isomers are the most potent isomers among the CLA mixture as suggested by others.;In conclusion, results in this study show that CLA inhibits the Delta6-desaturation and elongation of LA, and Delta5-desaturation of DGLA. The combined inhibitory effects of CLA on these three steps modulate n-6 PUFA metabolism and hence reduce the production of AA. This, in turn reduces the amount of AA entering the cyclooxygenase/lipoxygenase pathways, resulting in a possible reduction in the biosynthesis of pro-inflammatory eicosanoids in mammalian cells.