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We examined herein the functional linkage of enzymes regulating the initial, intermediate, and terminal steps of PG biosynthesis to provide PGs in rat peritoneal macrophages stimulated with LPS and/or A23187. Quiescent cells stimulated with A23187 produced thromboxane B2 (TXB2) in marked preference to PGE2 within 30 to 60 min (constitutive immediate response), which was mediated by preexisting cytosolic phospholipase A2 (cPLA2), cyclooxygenase-1 (COX-1), and TX synthase. Cells treated with LPS predominantly produced PGE2 during culture for 3 to 24 h (delayed response), where cPLA2 and secretory PLA2 functioned cooperatively with inducible COX-2, which was, in turn, coupled with inducible PGE2 synthase. Cells primed for 12 h with LPS and stimulated for 30 min with A23187 produced PGE2 in marked preference to TXB2 (induced immediate response), in which three inducible enzymes, cPLA2, COX-2, and PGE2 synthase, were functionally linked. Preferred coupling of the two inducible enzymes, COX-2 and PGE2 synthase, was further confirmed by the ability of LPS-treated cells to convert exogenous arachidonic acid to PGE2 optimally at a time when both enzymes were simultaneously induced. These results suggest that distinct PG biosynthetic enzymes display segregated functional coupling following different transmembrane stimulation events even when enzymes that catalyze similar reactions in vitro coexist in the same cells.
Naraba et al. (Sun,) studied this question.