Jeanette H. W. Leusen^1,2, Arthur J. Verhoeven¹ and Dirk Roos^1,3

¹ Central Laboratory of the Netherlands Red Cross Blood Transfusion Service and Laboratory for Experimental and Clinical Immunology, University of Amsterdam, Plesmanlaan 125, 1066 CX Amsterdam, The Netherlands.

²Department of Pediatrics, Emma's Children Hospital, Academic Medical Center, Amsterdam, The Netherlands.

Received 04/15/96; Accepted 05/05/96; On-line 07/01/96

The NADPH oxidase is a multi-component enzyme, localized in the plasma membrane of phagocytic leukocytes (2). It accepts electrons from NADPH at the cytosolic side of the membrane and donates these to molecular oxygen at the other side of the membrane, either on the outside of the cells or in the phagosome that contains ingested micro-organisms (3). In this way, a one-electron reduction of oxygen to superoxide anion is catalyzed, at the expense of NADPH.

2 O₂ + NADPH ---> 2 O₂^{· -} + NADP⁺+ H⁺

The superoxide produced is subsequently converted to hydrogen peroxide (H₂O₂), hypochlorous acid (HOCl) and other microbicidal products.

In resting, non-phagocytosing leukocytes, the NADPH oxidase is inactive. In this form, the enzyme components are localized in different parts of the cell. Phagocyte activation, e.g. by binding of opsonized micro-organisms to cell surface receptors, leads to assembly of the active enzyme complex.

2.1. Mechanisms of activation of the oxidase

The oxidase can be activated by receptor-mediated and by receptor-independent mechanisms. Typical receptor-dependent stimuli are the complement fragment C5a, the chemotactic tripeptide N-formyl-Met-Leu-Phe (fMLP)(for reviews see (4, 5)), and immune complexes (for reviews, see (6-9)). Receptor-independent stimuli include long-chain unsaturated fatty acids and phorbol 12-myristate 13-acetate (PMA). Oxidase activation by receptor-mediated stimuli usually lasts less than 5 min, while receptor-independent stimuli activate the enzyme for a much longer period, but only when the stimulus remains present. It appears that in intact cells the activated oxidase is undergoing a continuous process of activation and deactivation.