Modulation of prostaglandin H synthase activity by conjugated linoleic acid (CLA) and specific CLA isomers

Conjugated linoleic acid (CLA) has been shown to inhibit tumorigenesis in a nimal models and is cytostatic to numerous cell lines in vitro. However, th e mechanism of action is unknown. in the current study, we determined the e ffects of CLA and specific isomers of CLA on the rate of oxygenation of ara chidonic acid by prostaglandin H synthase (PGHS) in ram seminal vesicle mic rosomes. The enzyme was incubated with 0.1 to 100 muM CLA or specific isome rs of CLA for 2 min prior to the addition of 44 to 176 CIM arachidonate. Th e isomers tested were 9(E),11(E) CLA; 9(Z),11(E) CLA; (Z),11(Z) CLA, and 10 (E), 12(Z) CLA. For a positive inhibitor control, flurbiprofen was used at 0.75 to 2.50 muM. Enzyme activity was assessed by measuring the rate of oxy gen consumption. Inclusion of CLA or specific isomers of CLA in the incubat ion mixtures inhibits PGHS. The efficacy differs for each isomer, with the 9(Z), 11(E) CLA isomer being the most effective and the 9(Z),11(E) CLA isom er being the least effective inhibitor among the four CLA isomers tested. T he K,values obtained by Dixon replots range from 18.7 muM for the most effe ctive isomer, 9(Z),11(E) CLA, to 105.3 muM for the least effective isomer, 9(Z),11(Z) CLA. The K-i value for flurbiprofen with ram seminal vesicle mic rosomes was 0.33 muM. As the concentration of arachidonate was increased, t he CLA-dependent inhibition of PGHS decreased, suggesting competitive inhib ition. The results of this study demonstrate the potential of CFA and speci fic isomers of CLA to modulate prostaglandin biosynthesis.