The Respiratory Burst

Free radicals are chemical species with one or more unpaired electrons in their outer orbital. Their production is essential to normal metabolism but they are theoretically destructive unless tightly controlled. 

Reactive oxygen species are generated in cells by both enzymatic and nonenzymatic reactions. They should be generated at the cell surface, as a response against the stimulus caused by the presence of a pathogen. 

Redox reactions are essential for the function of cell membranes (Del Castillo-Olivares et al. 2000). It should be stressed that every bioenergetic-ally competent cell membrane does contain redox systems (Skulackev 1988). A plasma membrane electron transport or redox system has been found in every living cell tested. Voegtlin et al. (1925) examined a relation between the redox state and cancer. The redox potential of cytochrome bsss.. a component of NADPH oxidase, is -245 mV. This is atypically low for a cytochrome b, but this fact enables the reduction of oxygen to snperoxide (Cross 

---

CC, and one CX3C and XC chemokine receptors have been cloned so far [2]. Receptor binding initiates a cascade of intracellular events mediated by the receptor-associated heterotrimeric G-proteins. These G-protein subunits trigger various effector enzymes that lead to the activation not only of chemotaxis but also to a wide range of fimctions in different leukocytes such as an increase in the respiratory burst, degranulation, phagocytosis, and lipid mediator synthesis. 

NADPH-oxidase j 2O2 + NADPH 20p + NADP -F H+ Key component of the respiratory burst Deficient in chronic granulomatous disease... 

The Respiratory Burst of Phagocytic Cells Involves NADPH Oxidase Helps Kill Bacteria... 

The electron transport chain system responsible for the respiratory burst (named NADPH oxidase) is composed of several components. One is cytochrome 6558, located in the plasma membrane it is a heterodimer, containing two polypeptides of 91 kDa and... 

Leukocytes are activated on exposure to bacteria and other stimuh NADPH oxidase plays a key role in the process of activation (the respiratory burst). Mutations in this enzyme and associated proteins cause chronic granulomatous disease. 

Polymorphonuclear leucocytes (PMNs) employ a system comprising myeloperoxidase, hydrogen peroxide, and a halide factor to kill microorganisms and tumour cells. This process is sometimes loosely called the respiratory burst , which refers to the sudden rise in oxygen consumption by the phagocytosing neutrophils that is independent of the mitochondrial electron transport chain. 

Chronic granulomatous disease is a rare inherited disorder characterized by the failure of neutrophils, eosinophils, monocytes and macrophages to produce the respiratory burst (Curnutte and Babior, 1987) this leads to recurrent bacterial and fungal infections often starting within the first year of life. 

Baggiolini, M. and Thelen, M. (1991). The phagocytes and the respiratory burst. In Oxidative Stress (ed. H. Sies) pp. 399-420. Academic Press, London. 

The mammalian immune system protects the body from infection by many complex strategies. The most vigorous defense is performed by white blood cells known as granulocytes. These cells consume oxygen in response to microbial infections. This oxidative process, called the respiratory burst, has recently been proven to produce stabilized hvDOchlorite antimicrobials (bredominantlv bv neutroDhils) and stabilized... 

At the same time the interaction of superoxide with MPO may affect a total superoxide production by phagocytes. Thus, the superoxide adduct of MPO (Compound III) is probably quantitatively formed in PMA-stimulated human neutrophils [223]. Edwards and Swan [224] proposed that superoxide production regulate the respiratory burst of stimulated human neutrophils. It has also been suggested that the interaction of superoxide with HRP, MPO, and LPO resulted in the formation of Compound III by a two-step reaction [225]. Superoxide is able to react relatively rapidly with peroxidases and their catalytic intermediates. For example, the rate constant for reaction of superoxide with Fe(III)MPO is equal to 1.1-2.1 x 1061 mol 1 s 1 [226], and the rate constants for the reactions of Oi and HOO with HRP Compound I are equal to 1.6 x 106 and 2.2 x 1081 mol-1 s-1, respectively [227]. Thus, peroxidases may change their functions, from acting as prooxidant enzymes and the catalysts of free radical processes, and acquire antioxidant catalase properties as shown for HRP [228] and MPO [229]. In this case catalase activity depends on the two-electron oxidation of hydrogen peroxide by Compound I. 

Torres MA, Dangl JL (2005) Functions of the respiratory burst oxidase in biotic interactions, abiotic stress and development. Curr Opin Plant Biol 8 397 403 Torres MA, Jones JD, Dangl JL (2006) Reactive oxygen species signaling in response to pathogens. Plant Physiol 141 373-378... 

Figure 1.4. Recognition of bacteria by neutrophils. Invading bacteria are opsonised by serum proteins, such as complement fragments (e.g. C3b) and immunoglobulins. The plasma membranes of neutrophils possess receptors for these opsonins (e.g. Fc receptors and complement receptors). Thus, occupancy of these opsonin receptors triggers phagocytosis and activates events such as the respiratory burst and degranulation. Note that the receptors and opsonins are not drawn to scale.

Experiments by Peter Elsbach and colleagues in the 1970s showed that although E. coli lost viability very quickly after incubation with neutrophils, these non-viable organisms still retained several important biochemical functions, such as membrane transport and macromolecular biosynthesis. As these functions are associated with the inner plasma membrane of the bacteria, these observations suggested that the lethal hit on E. coli by neutrophils occurred on the outer membrane. Because disrupted neutrophils also affected the bacteria in this way, it was concluded that the process was independent of the respiratory burst hence these workers investigated the granule proteins for the source of this activity (reviewed in Elsbach Weiss, 1983). 

C5a and C5a des Arg stimulate aerobic glycolysis, hexose monophosphate shunt activity, glucose uptake and the respiratory burst of human neutrophils. All of these processes are stimulated in neutrophil suspensions incubated in the absence of cytochalasin B, but the responses are considerably enhanced if this inhibitor of microtubule assembly is present. Stimulated rates of oxidative metabolism are maximal within 2 min of addition of peptides, with half-maximal responses obtained at 30-60 nM C5a and 1-3 pM C5a des Arg. 

A large number of cytokines generated during an inflammatory response can affect neutrophil function. Some of these cytokines, such as G-CSF and GM-CSF, can affect the rate of biosynthesis of mature neutrophils in the bone marrow they can also affect the function of mature cells by priming certain functions (such as the respiratory burst, degranulation and expression of some plasma membrane receptors). These effects are described in detail in Chapters 2 and 7, respectively the present chapter briefly describes some the properties of cytokines known to affect the function of mature neutrophils. 

For many years, it has been recognised that incubation of platelets with neutrophils enhances some fMet-leu-Phe-stimulated functions, such as aggregation and the respiratory burst. The platelet-derived factor responsible for this effect does not stimulate neutrophils in the absence of external stimuli (e.g. fMet-Leu-Phe), and the effects may be mimicked by the addition of ATP or ADP to neutrophil suspensions. 

The respiratory burst The generation of reactive oxygen metabolites and their role in microbial killing... 

Before we discuss how neutrophils (and other professional phagocytes) generate O2 and other oxygen metabolites during the respiratory burst, let us first consider why they produce such compounds. If these metabolites are produced in order to kill phagocytosed microorganisms, why and how are they toxic The answer comes from an examination of the chemistry of the oxygen molecule itself. 

---