Conversion of leukotrienes A4 to C4 in cell-free systems

A procedure for assaying leukotriene C4 synthase activity in cell-free extracts has been presented. Leukotriene A4 methyl ester was as active a substrate as leukotriene A4 (Na salt) for the synthesis. The methyl ester is the substrate of choice, because (1) it is more stable than the sodium salt, (2) it is not a substrate of epoxide hydrolase for leukotriene B4 synthesis, and (3) it gives a lower blank than an equimolar concentration of leukotriene A4. The enzyme activity in rat liver, guinea pig and human lungs, and human nasal polyp was chiefly membrane-bound, although the cytosol contained some activity.