Differential metabolism of exogenous and endogenous arachidonic acid in human neutrophils

Leukotrienes can be produced by cooperative interactions between cells in w hich, for example, arachidonate derived from one cell is oxidized to leukot riene A(4) (LTA(4)) by another and this can then be exported for conversion to LTB4 or cysteinyl leukotrienes (cys-LTs) by yet another. Neutrophils do not contain LTC4 synthase but are known to cooperate with endothelial cell s or platelets (which do have this enzyme) to generate cys-LTs, Stimulation of human neutrophils perfusing isolated rabbit hearts resulted in producti on of cys-LTs, whereas these were not seen with perfused hearts alone or is olated neutrophils. In addition, the stimulated, neutrophil-perfused hearts generated much greater amounts of total LTA(4) products, suggesting that t he hearts were supplying arachidonate to the neutrophils and, in addition, that this externally derived arachidonate was preferentially used for expor ted LTA(4) that could be metabolized to cys-LTs by the coronary endothelium . Stable isotope-labeled arachidonate and electrospray tandem mass spectrom etry were used to differentially follow metabolism of exogenous and endogen ous arachidonate, Isolated, adherent neutrophils at low concentrations (to minimize transcellular metabolism between them) were shown to generate high er proportions of nonenzymatic LTA(4) products from exogenous arachidonate (deuterium-labeled) than from endogenous (unlabeled) sources. The endogenou s arachidonate, on the other hand, was preferentially used for conversion t o LTB4 by the LTA(4) hydrolase, This result was not because of saturation o f the LTA(4) hydrolase, because it occurred at widely differing concentrati ons of exogenous arachidonate, Finally, in the presence of platelets (which contain LTC4 synthase), the LTA(4) synthesized from exogenous deuterium-la beled arachidonate was converted to cys-LTs to a greater degree than that f rom endogenous sources. These experiments suggest that exogenous arachidona te is preferentially converted to LTA(4) for export (not intracellular conv ersion) and raises the likelihood that there are different intracellular pa thways for arachidonate metabolism.