A Ca2+-stimulated adenosine triphosphatase in Golgi-enriched membranes of lactating murine mammary tissue.

A membrane fraction isolated from lactating murine mammary tissue and enriched for the Golgi membrane marker enzyme galactosyltransferase exhibited Ca2+-stimulated ATPase activity (Ca-ATPase) in 20 microM-free Mg2+ and 10 microM-MgATP, with an apparent Km for Ca2+ of 0.8 microM. Exogenous calmodulin did not enhance Ca2+ stimulation, nor could Ca-ATPase activities be detected in millimolar total Mg2+ and ATP. When assayed with micromolar Mg2+ and MgATP the Ca-ATPases of skeletal-muscle sarcoplasmic reticulum and of calmodulin-enriched red blood cell plasma membranes were half-maximally activated by 0.1 microM- and 0.6 microM-Ca2+ respectively. All three Ca-ATPases were inhibited by similar micromolar concentrations of trifluoperazine, but the Golgi activity was unaffected by quercetin in concentrations which completely inhibited both the sarcoplasmic-reticulum and red-blood-cell enzymes. The results are consistent with the hypothesis that the high-affinity Ca-ATPase is responsible for the ATP-dependent Ca2+ transport exhibited by Golgi-enriched vesicles derived from lactating mammary gland [Neville, Selker, Semple & Watters (1981) J. Membr. Biol. 61, 97-105; West (1981) Biochim. Biophys. Acta 673, 374-386].     

Manipulation of platelet aggregation by prostaglandins and their fatty acid precursors: pharmacological basis for a therapeutic approach

Addition of the one-, two- or three- series endoperoxide to human platelet-rich plasma tend to suppress aggregation, through the action of their respective non-enzymatic breakdown products PGE1, PGD2, or PGD3 all of which elevate cyclic AMP levels. On the other hand, these stable primary products do not arise in appreciable amounts from intrinsic endoperoxides generated from either endogenous or exogenous free fatty acids. 5,8,11,14,17-Eicosapentaenoic acid (EPA) suppresses arachidonic acid (5,8,11,14-eicosatetraenoic acid) conversion by cyclooxygenase (as well as lipoxygenase) to aggregatory metabolites in platelets. Exogenously added EPA was capable of inhibiting PRP aggregation induced either by exogenous or endogenous (released by ADP or collagen) arachidonate. The hypothetical combination of an EPA-rich diet and a thromboxane synthetase inhibitor might abolish production of the pro-aggregatory species, thromboxane A2, and enhance formation of the anti-aggregatory metabolite, prostacyclin. Whereas EPA is not detectably metabolized by platelets, dihomo-gamma-linolenic acid (8,11,14-eicosatrienoic acid) is primarily converted by cyclooxygenase and thromboxane synthetase into the inactive metabolite, 12-hydroxyheptadecadienoic (HHD) acid. Pretreatment of human platelet suspensions with the thromboxane synthetase inhibitor imidazole unmasks the aggregatory property of PGH1 and DLL which was partially compromised by the PGE1 formed. The combination of the thromboxane synthetase inhibitor and an adenylate cyclase inhibitor unmasks a complete irreversible aggregation by DLL or PGH1. The basis of a dietary strategy that replaces AA with DLL must rely on the production by the platelet of an inactive metabolite (HHD) rather than thromboxane A2.