The role of Delta(9)-desaturase in the production of cis-9, trans-11 CLA.

Biomedical studies with animal models have demonstrated that cis-9, trans-11 conjugated linoleic acid (CLA), the predominant isomer found in milk fat from dairy cows, has anticarcinogenic effects. We recently demonstrated endogenous synthesis of cis-9, trans-11 CLA from ruminally derived trans-11 C18:1 by Delta(9)-desaturase in lactating dairy cows. The present study further examined endogenous synthesis of cis-9, trans-11 CLA and quantified its importance by increasing substrate supply using partially hydrogenated vegetable oil (PHVO) as a source of trans-11 C18:1 and blocking endogenous synthesis using sterculic oil (SO) as a source of cyclopropene fatty acids which specifically inhibit Delta(9)-desaturase. Four cows were abomasally infused with 1) control, 2) PHVO, 3) SO, and 4) PHVO+SO in a 4 x 4 Latin square design. With infusion of PHVO, cis-9, trans-11 CLA was increased by 17% in milk fat. Consistent with inhibition of desaturase, SO treatments increased milk fat ratios for the fatty acid pairs effected by Delta(9)-desaturase, C14:0/cis-9 C14:1, C16:0/cis-9 C16:1, and C18:0/cis-9 C18:1. The role of endogenous synthesis of CLA was evident from the 60-65% reduction in cis-9, trans-11 CLA which occurred in milk fat with SO treatments. cis-9 C14:1 originates from desaturation of C14:0 by Delta(9)-desaturase and can be used to estimate the extent of SO inhibition of Delta(9)-desaturase. When this correction factor was applied, endogenous synthesis was estimated to account for 78% of the total cis-9, trans-11 CLA in milk fat. Thus, endogenous synthesis was the major source of cis-9, trans-11 CLA in milk fat of lactating cows.