Inactivation of NADPH oxidase from human neutrophils by affinity labeling with pyridoxal 5'-diphospho-5'-adenosine

Perfluoroalkylphosphocholines are poor protein-solubilizing surfactants, as tested with neutrophil plasma membranes

We have tested the membrane-protein solubilizing properties of two perfluoroalkylphosphocholines. These compounds belong to a series of fluorinated amphiphiles which are being investigated as potential stabilizing agents for a variety of fluorocarbon-based systems. We are particularly interested in cytochrome b558 from phagocytes, the redox component of NADPH oxidase. Its heavy subunit is believed to carry binding sites for NADPH and FAD. Nevertheless, when the cytochrome is purified in the presence of classical detergents, it carries no FAD. This could be due to a delipidating, denaturing effect of these detergents (octyl glucoside, Triton, etc). The first perfluoroalkyphosphocholine, C8F17(CH2)2O-P(O2-)-O(CH2)2N+(CH3)3(F8C2PC), extracted about as much protein from neutrophil plasma membranes into a 100,000 g supernatant as octyl glucoside. The second compound, C8F17(CH2)11O-P(O2-)-O(CH2)2N+(CH3)3(F8C11PC), was less efficient. We found that flavin was still protein-bound in the crude F8C2PC extract at a FAD to heme ratio of about 1, and a good NADPH oxidase activity was obtained without addition of exogenous FAD, even after dialysis or gel filtration, whereas dialysis eliminated most of the FAD from the octyl glucoside extracts. These experiments appeared to make F8C2PC an interesting membrane-solubilizing agent. Nevertheless, no protein in the F8C2PC extract could be adsorbed on the chromatographic supports normally used for purification. After dilution of the extract and addition of 15 mM octyl glucoside, some of the proteins, such as myeloperoxidase, could be adsorbed (and eluted), but not cytochrome b558. Freeze-fracture electron microscopy showed that the F8C2PC extracts contained numerous vesicles and aggregates of small shapeless particles. Higher centrifugal forces sedimented most proteins of the 100,000 g supernatant. As a check, the effect of F8C2PC was tested on sarcoplasmic reticulum vesicles, the behavior of which with respect to the usual non-denaturating detergents has been well studied. There was little, if any, solubilization. We conclude that, although supernatants of F8C2PC extracts of neutrophil membranes are optically clear, proteins are not really solubilized. This result is in keeping with the absence of lytic effects of F8C2PC on erythrocyte membranes.

Aminoacyl chloromethanes as tools to study the requirements of NADPH oxidase activation in human neutrophils

Previous studies from this laboratory described the kinetic characteristics of the inhibition by tosylphenylalanine chloromethane (TosPheCH2Cl) on superoxide anion production by human neutrophils (PMN) stimulated with a phorbol ester (PMA). In this study we present further evidence concerning the potential role of the chloromethane target in the normal cellular activation of NADPH oxidase. When PMN are treated with TosPheCH2Cl and subsequently PMA, or with the two reagents in the reverse order, the inhibition of superoxide production by the intact cells is still present in a particulate NADPH oxidase fraction prepared from these cells. Nevertheless, when cells incubated only with the chloromethane and not with PMA are disrupted, both their cytosolic and membrane fractions are fully competent in the cell-free activation assay. Thus, the chloromethane target has a role in NADPH oxidase activation exclusively at the cellular level. This observation constitutes additional evidence in favour of the idea that activation in the cell-free system reflects only partially the events which occur in the cells. When cells are activated with PMA, their cytosol displays a loss of activating capacity in the cell-free activation assay in the presence of arachidonate, as was shown before with SDS as activator [Ambruso, D. R., Bolsher, B. G. J. M., Stockman, P. M., Verhoeven, A. J. & Roos, D. (1990) J. Biol. Chem. 265, 924-930]. This phenomenon was shown to arise most probably from the translocation of cytosolic factors to the membrane, resulting in a depleted cytosol. When superoxide production was inhibited by cell treatment with TosPheCH2Cl, either before or after activation with PMA, the cytosol from inhibited cells showed a recovery of activation capacity in the cell-free system. This effect probably results from TosPheCH2Cl inhibiting the translocation of the cytosolic factors when added before PMA. This results in an insufficient activation at the membrane level, which was previously considered as an inhibition. The effect of TosPheCH2Cl, when added after PMA, can best be explained again as an inhibition of translocation in the frame of the continuous replenishment-deactivation hypothesis proposed by Akard et al. [Akard, L. P., English, D. & Gabig, T. G. (1988) Blood 72, 322-327]. Thus, TosPheCH2Cl is apparently a promising new tool for studying the activation of NADPH oxidase at the cellular level.(ABSTRACT TRUNCATED AT 400 WORDS)

Critical assessment of the presence of an NADPH binding site on neutrophil cytochrome b558 by photoaffinity and immunochemical labeling

The presumed NADPH dehydrogenase function of the heterodimeric cytochrome b558 in the neutrophil oxidase complex has been investigated by combined photoaffinity labeling and immunoblot analysis of membrane proteins from bovine neutrophils. The photoaffinity probe was a radiolabeled analog of NADPH, [4-[N-(4-azido-2-nitrophenyl)[3H]amino]butyryl]NADPH ([3H]azido-NADPH), and the antibodies were directed against the C-terminal regions of the two subunits of cytochrome b558. Plasma membrane vesicles obtained by differential centrifugation of bovine neutrophil homogenates were routinely used as a source of NADPH oxidase. They were permeabilized by sodium deoxycholate to facilitate the access of NADPH or its azido analog to the totality of the specific binding sites. In the absence of light, azido-NADPH behaved as a competitive inhibitor of NADPH oxidase with a Ki of 6 microM, and was able to bind to high-affinity specific binding sites with a Kd of 5-6 microM, indicating a higher affinity of the oxidase for the photoprobe than for the substrate NADPH (KM = 30-40 microM). Upon photolabeling, the oxidase was fully inactivated. Following resolution of the membrane proteins by SDS-PAGE, a predominant photolabeled protein band of 80-100 kDa was revealed, which coincided with the large subunit (beta) of cytochrome b558 identified by immunoblot in a parallel gel. The enzymatic deglycosylation of photolabeled neutrophil membranes shifted the masses of both the photolabeled band and the immunoreactive beta subunit from 80-100 to 55-65 kDa in accordance with the glycoprotein nature of the beta subunit.(ABSTRACT TRUNCATED AT 250 WORDS)