A trifluoromethyl ketone analogue of arachidonic acid in which the COOH group is replaced with COCF3 (AACOCF3) was prepared and found to be a tight- and slow-binding inhibitor of the 85-kDa cytosolic human phospholipase A2 (cPLA2). Enzyme inhibition was observed when AACOCF3 was tested in assays using either phospholipid vesicles or phospholipid/Triton X-100 mixed micelles. The fact that the inhibition developed over several minutes in both assays establishes that AACOCF3 inhibits by direct binding to the enzyme rather than by decreasing the fraction of enzyme bound to the substrate interface. From the measured values of the inhibitor association and dissociation rate constants, an upper limit of the equilibrium dissociation constant for the Ca(2+).AACOCF3.PLA2 complex of 5 x 10(-5) mole fraction was obtained. Thus, detectable inhibition of cPLA2 by AACOCF3 occurs when this compound is present in the assay at a level of one inhibitor per several thousand substrates. Arachidonic acid analogues in which the COOH group is replaced by COCH3, CH(OH)CF3, CHO, or CONH2 did not detectably inhibit the cPLA2. The arachidonyl ketones AACOCF2CF3 and AACOCF2Cl were found by 19F NMR to be less hydrated than AACOCF3 in phospholipid/Triton X-100 mixed micelles, and compared to AACOCF3 these compounds are also weaker inhibitors of cPLA2. In keeping with the fact that cPLA2 displays substrate specificity for arachidonyl-containing phospholipids, the arachidic acid analogue C19H39COCF3 is a considerably less potent inhibitor compared to AACOCF3.(ABSTRACT TRUNCATED AT 250 WORDS)

A MAb (TP-2) directed against human cholesteryl ester transfer protein (CETP) has been applied to the development of a competitive solid-phase RIA. Experiments with immobilized CETP have shown that upon incubation with plasma or HDL in the presence of Tween (0.05%) apo A-I (but not apo A-II) binds to CETP while TP-2 binding to CETP is concomitantly decreased. With high detergent concentration (0.5% Triton), the interference is eliminated and a specific RIA in which all plasma CETP fractions have the same affinity can be obtained. Plasma levels of CETP, apo A-I, lipids, and lipoproteins were measured in 50 normolipemic, healthy subjects of both sexes. CETP levels varied nearly fourfold with a mean value of 1.7 gg/ml. CETP was positively correlated only with apo A-I (r = 0.38) and HDL-triglyceride (r = 0.39). In 29 other normolipemic subjects, where several apolipoproteins were also measured, significant correlations of CETP with apo A-I (0.41), apo E (0.43), and HDL-cholesterol (0.41) were observed, but there was no significant relationship between CETP and either apo A-II, B, or D. In other experiments CETP was shown to be present mostly in HDL3 and VHDL, to display exclusively an a2-electrophoretic migration, and to occur within discrete particles ranging in size from 129 to 154 kD. In conclusion, the association of CETP with apo A-I-containing lipoproteins probably explains the correlation between CETP and apo A-I levels. The relationship between CETP and apo E suggests either a common metabolism or a specific cooperative role in cholesterol ester transport for these proteins. (J. Clin. Invest. 1990. 85:10-17.) cholesterol * cholesteryl ester transfer protein-high-density lipoprotein-lipoprotein

The distribution of cytosolic phospholipase A, (cPLA,), arachidonate 5-lipoxygenase, and 5-lipoxygenase-activating protein (5-LAP) was investigated in subcellular fractions of human neutrophils disrupted by three techniques. As determined by immunoblot analysis, the bulk of cPLA, and 5-lipoxygenase was detected in cytosolic fractions of unstimulated neutrophils disrupted by sonication or cavitation. After cell stimulation with the calcium ionophore A231 87, both proteins accumulated primarily in nucleicontaining fractions; this accumulation was accompanied by a loss of these enzymes from cytosolic fractions. Further resolution of nuclear fractions revealed that 5-lipoxygenase and cPLA, were localized in a fraction that contained nuclear membranes. In comparison, 5-LAP was localized to the nuclearmembrane fraction of resting and activated neutrophils, as determined by immunoblotting and photoaffinity labeling. In agreement with the immunoblot data, A231 87 stimulation markedly enhanced 5-lipoxygenase enzymatic activity in the nuclear-membrane fraction, which was accompanied by decreased cytosolic 5-lipoxygenase activity. Similarly, neutrophil activation caused increased phosphorylation of cPLA,, a process that is known to result in enhanced catalytic activity. Our data demonstrate that in activated human neutrophils, the key proteins involved in leukotriene synthesis colocalize at the nuclear membrane, in a catalytically active state.

Arachidonyl trifluoromethyl ketone (AACOCF3) is a slow- and tight-binding inhibitor of the human cytosolic phospholipase A2 (cPLA2) [Street et al. (1993) Biochemistry 32, 5935]. 19F and 13C NMR experiments have been carried out to elucidate the structure of the cPLA2.AACOCF3 complex. One mole of AACOCF3 per mole of enzyme is tightly bound in the active site while excess molar equivalents of the inhibitor associate loosely and nonspecifically with hydrophobic regions of the protein. Incubation of the cPLA2.AACOCF3 complex with a 10-fold molar excess of a structurally related inhibitor allows the slow dissociation of the enzyme-inhibitor complex to be followed with 19F NMR. These results establish that the bound inhibitor is in slow exchange with the free ligand and that inhibition of the cPLA2 by AACOCF3 is not due to irreversible modification of the protein. AACOCF3 labeled with 13C at the carbonyl position was used to determine the nature of the bound inhibitor species. A comparison of the 13C NMR chemical shift value obtained from labeled enzyme-inhibitor complex (delta c 101.0 ppm) with the chemical shift values obtained from model compounds suggests that the enzyme-bound inhibitor species is a charged hemiketal. These results are very similar to those obtained previously with alpha-chymotrypsin and a peptidyl trifluoromethyl ketone inhibitor [Liang, T.-C., & Abeles, R. H. (1987) Biochemistry 26, 7603] and, by analogy with the serine proteases, a structural model for the cPLA2.AACOCF3 complex is proposed.

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