Human myeloperoxidase catalyzes an oscillating peroxidase-oxidase reaction

NADH oxidase activity of indoleamine 2,3-dioxygenase

The heme enzyme indoleamine 2,3-dioxygenase (IDO) was found to oxidize NADH under aerobic conditions in the absence of other enzymes or reactants. This reaction led to the formation of the dioxygen adduct of IDO and supported the oxidation of Trp to N-formylkynurenine. Formation of the dioxygen adduct and oxidation of Trp were accelerated by the addition of small amounts of hydrogen peroxide, and both processes were inhibited in the presence of either superoxide dismutase or catalase. Anaerobic reaction of IDO with NADH proceeded only in the presence of a mediator (e.g. methylene blue) and resulted in formation of the ferrous form of the enzyme. We propose that trace amounts of peroxide previously proposed to occur in NADH solutions as well as solid NADH activate IDO and lead to aerobic formation of superoxide and the reactive dioxygen adduct of the enzyme.

On the mechanism of oscillations in neutrophils

We have investigated the regulation of the oscillatory generation of H(2)O(2) and oscillations in shape and size in neutrophils in suspension. The oscillations are independent of cell density and hence do not represent a collective phenomena. Furthermore, the oscillations are independent of the external glucose concentration and the oscillations in H(2)O(2) production are 180 degrees out of phase with the oscillations in NAD(P)H. Cytochalasin B blocked the oscillations in shape and size whereas it increased the period of the oscillations in H(2)O(2) production. 1- and 2-butanol also blocked the oscillations in shape and size, but only 1-butanol inhibited the oscillations in H(2)O(2) production. We conjecture that the oscillations are likely to be due to feedback regulations in the signal transduction cascade involving phosphoinositide 3-kinases (PI3K). We have tested this using a simple mathematical model, which explains most of our experimental observations.

Biofilm effects on the peroxidase-oxidase reaction

In experiments on the kinetics of the peroxidase-oxidase oscillatory reaction in pH 5.l acetate buffer, biofilms form in less than 48 h on the quartz reactor surface. The nominally homogeneous peroxidase system shows dynamical changes in response to this biofilm growth, partially explaining subtle differences among dynamics observed over time and between laboratories. Kinetics data and model computations are correlated with micrographs of biofilm formation. It is evident that bare quartz also interacts with reaction species, so that the surface area-to-volume ratio is an important parameter on which observed dynamics depend.

Myeloperoxidase accumulates at the neutrophil surface and enhances cell metabolism and oxidant release during pregnancy

Pregnancy is a unique immunological state. Pregnancy neutrophils differ from those of non-pregnant women as they cannot be fully activated for oxidant production, but yet have higher levels of unstimulated oxidant production. Although reduced activation is due to decreased hexose monophosphate shunt activity, the mechanism enhancing basal oxidant levels is unknown. We hypothesize that myeloperoxidase (MPO) trafficking affects the basal oxidant release by maternal neutrophils. Immunofluorescence microscopy has demonstrated MPO at the surface of pregnancy neutrophils, whereas non-pregnancy cells do not exhibit surface MPO. Adherent pregnancy neutrophils were characterized by high-amplitude metabolic oscillations, which were blocked by MPO inactivation. Conversely, metabolic oscillatory amplitudes of control neutrophils were heightened by incubation with PMA or exogenous MPO. Importantly, MPO decoration of cell surfaces and high-amplitude metabolic oscillations were observed for neutrophils from pregnant but not from non-pregnant mice. However, cells from pregnant MPO knockout mice did not exhibit MPO expression or high-amplitude metabolic oscillations. Unstimulated neutrophils from pregnant women were found to release reactive oxygen metabolites (ROM) and reactive nitrogen intermediates (RNI), but cells from non-pregnant women did not. MPO inhibition returned ROM and RNI formation to non-pregnant levels. Hence, MPO trafficking influences metabolic activity and oxidant production in pregnancy.