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

Abstract

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)