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Fluorescence Spectroscopic Detection of Mitochondrial Flavoprotein Redox

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Fluorescence Spectroscopic Detection of Mitochondrial Flavoprotein Redox Eur Biophys J (2004) 33: 291–299 DOI 10.1007/s00249-003-0361-4 ARTICLE Andrei Kindzelskii Æ Howard R. Petty Fluorescence spectroscopic detection of mitochondrial flavoprotein redox oscillations and transient reduction of the NADPH oxidase-associated flavoprotein in leukocytes Received: 27 May 2003 / Accepted: 27 September 2003 / Published online: 23 October 2003 Ó EBSA 2003 Abstract Steady-state and time-resolved fluorescence oxidase-associated flavoprotein undergoes a periodic spectroscopy and fluorescence microscopy of leukocyte transient reduction of about 54±2 ms in living cells. flavoproteins have been performed. Both living human This finding is consistent with prior studies indicating peripheral blood monocytes and neutrophils have been that propagating substrate (NADPH) waves periodi- utilized as experimental models, as the former relies cally promote electron transport across the NADPH much more heavily on mitochondrial metabolism for oxidase. energy production than the latter. We confirm previ- ous studies indicating that cellular flavoproteins ab- Keywords Flavoprotein Æ Leukocyte Æ Mitochondria Æ sorb at 460 nm and emit at 530 nm, very similar to NADPH oxidase Æ Fluorescence spectroscopy that of the FAD moiety. Furthermore, the emission properties of intracellular flavoproteins were altered by the metabolic inhibitors rotenone, antimycin A, azide, cyanide, DNP (2,4-dinitrophenol), and FCCP [car- Introduction bonyl cyanide p-(trifluoromethoxy)phenylhydrazone]. Kinetic studies revealed flavoprotein emission oscilla- Electron transport systems play vital roles in energy tions in both monocytes and neutrophils. The flavo- production and in the production of superoxide anions, protein intensity oscillations correlated with the an important reactive oxygen metabolite (ROM). physiological status of the cell and the nature of Flavoproteins participate in (1) mitochondrial electron membrane receptor ligation. Microscopy revealed the transport reactions of the NADH dehydrogenase presence of flavoprotein fluorescence in association (complex I of the respiratory chain) and (2) superoxide with the plasma membrane, intracellular granules and anion formation by the NADPH oxidase, a protein distributed throughout the cytoplasm, presumably found in granule and plasma membrane fractions of within mitochondria. Metabolic inhibitors such as leukocytes. Flavoproteins are also noteworthy because cyanide suggest that the plasma membrane and gran- they demonstrate autofluorescence in the oxidized, but ular components are cyanide-insensitive and therefore not reduced, state (Kunz 1986; Kunz and Kunz 1985; are likely associated with the flavoprotein component Kunz et al. 1993, 1997, 2002; Kuznetsov et al. 1998). of the NADPH oxidase, which is located in these two Thus, flavoprotein autofluorescence reflects electron compartments. This interpretation was found to be trafficking through the proteinsÕ active sites. Flavopro- consistent with structural localization of the NADPH tein fluorescence has been used extensively to monitor oxidase using an antibody molecule specific for this the metabolic status of cells. About one-half of the protein. Using peripheral blood neutrophils, which flavoprotein fluorescence is due to a-lipoamide dehy- display less active mitochondria, and time-resolved drogenase (a-LAD), a component of 2-oxoacid dehy- emission spectroscopy, we show that the NADPH drogenase multienzyme complexes in the mitochondrial matrix, whereas approximately one-quarter is due to the electron transport dehydrogenaseÕs flavin moiety (Kunz A. Kindzelskii Æ H. R. Petty (&) and Kunz 1985). The redox state of a-LADÕs flavin co- Departments of Ophthalmology and Visual Science and factor is in tight equilibrium with the mitochondrial Microbiology and Immunology, The University of Michigan NAD+/NADH pool. Therefore, the amount of flavo- School of Medicine, 1000 Wall Street, Ann Arbor, MI 48105, USA E-mail: [email protected] protein autofluorescence is inversely related to the Tel.: +1-734-6470384 amount of mitochondrial metabolic activity. This is Fax: +1-734-9363815 illustrated by the scheme (Kunz et al. 1997): 292 þ Primary dehydrogenases ! NADH ðred:; fluorescentÞ=NAD ðoxid:Þ!Respiratory chain ! O2 "# ð1Þ a À LAD À H2 ðred:Þ=a À LAD ðoxid:; fluorescentÞ When metabolic activity is high, NADH predomi- living cells. The implications of this behavior in host nates and non-fluorescent, reduced flavoprotein defense are considered. (a-LAD-H2) is abundant. Similarly, uncouplers of oxi- dative phosphorylation, such as 2,4-dinitrophenol (DNP) and carbonyl cyanide p-(trifluorometh- Materials and methods oxy)phenylhydrazone (FCCP), accelerate electron transport, thereby promoting flavoprotein oxidation and Reagents increased fluorescence. When metabolic activity is low + FMLP (N-formyl-met-leu-phe), diphenylene iodonium (DPI), (e.g., apoptosis), NAD predominates and oxidized, rotenone, antimycin A, cyanide, azide, DNP, and FCCP were fluorescent flavoprotein (a-LAD) predominates. In the obtained from Sigma (St. Louis, Mo.). presence of the membrane-permeable mitochondrial substrate octanoate and the cytochrome oxidase (com- plex IV) inhibitor cyanide (which ‘‘backs-up’’ electrons Cell preparation in the respiratory chain), the respiratory chain linked flavoproteins are predominantly reduced, decreasing Neutrophils and monocytes were isolated from peripheral blood using Ficoll-Hypaque (Sigma) density gradient centrifugation at flavoprotein fluorescence. Thus, the inversely related 300·g for 30 min at room temperature (Kindzelskii et al. 1997). autofluorescence intensities of NAD(P)H and flavopro- The viability of the isolated neutrophils was always >95%, as teins are metabolic barometers in living cells. assessed by trypan blue exclusion. Cells were suspended in HBSS In addition to mitochondrial flavoproteins, several (Hanks balanced salt solution) or PBS (phosphate-buffered saline) cell types such as monocytes and neutrophils also pos- (Life Technologies, Grand Island, NY). sess flavoproteins in association with the NADPH oxi- dase complex, which produces superoxide anions that Immunofluorescence staining participate in host defense mechanisms (Babior 1999). The NADPH oxidase is made up of multiple compo- After washing with PBS, adherent cells were fixed with cold 3.7% nents, including p47-phox, p67-phox, Rac, and p91- paraformaldehyde for 30 min. Cells were rinsed once, then fresh phox, which is a flavoprotein. gp91 has binding sites for 3.7% paraformaldehyde wash added for an additional hour. After washing with PBS, cells were permeabilized with 1% Brij-58 for NADPH, FAD, and heme. Although the properties of 30 min at room temperature, followed by washing and fixation purified NADPH oxidase components have been thor- with 3.7% paraformaldehyde for 30 min. Samples were labeled oughly studied in vitro, the operational nature and with a rabbit anti-gp91-phox antibody (Upstate Biotechnology, regulation of the oxidase in living cells is not well Lake Placid, NY) for 20 min at room temperature. Cells were washed then labeled with a rhodamine-conjugated mouse anti- understood. Recently, we have reported various aspects rabbit IgG for 30 min at room temperature followed by extensive of the substrate-level regulation of the NADPH oxidase. washing with PBS. For example, the activity of the oxidase may be regu- lated by the availability of its substrate NADPH (Kindzelskii and Petty 2002; Kindzelskii et al. 1997, Fluorescence imaging 2002; Petty 2000, 2001). Indeed, the oxidase is part of a A fluorescence microscope with a quartz condenser, quartz objec- larger dynamic regulatory system within neutrophils tives, and a mercury illumination source (Carl Zeiss, New York, which leads to changes in the amplitude of NADPH NY) were employed. The microscopeÕs bottom port was employed oscillations and the amplitude of ROM production for fluorescence detection to minimize the number of optical (Olsen et al. 2003). In addition, the blunted activation of components. This port was fiber-optically coupled to a cooled ) neutrophils from pregnant women may be traced to intensifier ( 20 °C), which was attached to a Peltier-cooled I-MAX-512 camera ()20 °C) (Princeton Instruments, Princeton, retrograde translocation of glucose-6-phosphate dehy- NJ). Image acquisition was controlled by a high-speed Princeton drogenase, which is part of the NADPH-producing ST-133 interface and a Stanford Research Systems (Sunnyvale, hexose monophosphate shunt (Kindzelskii et al. 2002). Calif.) DG-535 delay gate generator. For flavoprotein fluorescence Furthermore, using high-speed microscopy we have re- imaging, a filter set comprising a 455 nm DF70 excitation filter, a 520 nm DF40 emission filter, and a 495 nm long-pass dichroic cently shown that NAD(P)H waves travel along the long reflector was used. For rhodamine fluorescence imaging, a 540 nm axis of neutrophils and then release superoxide anions as DF20 excitation filter, a 590 nm DF30 emission filter, and a a pulse at the leading edge (Kindzelskii and Petty 2002). 560 nm long-pass dichroic reflector were used. In the present study we explore the redox state of flavoproteins in neutrophils and monocytes. In particu- lar, we show that the flavoprotein component of the Emission spectroscopy NADPH oxidase undergoes periodic reduction lasting The system described in the previous paragraph was used to per- 54 ms; this work provides compelling evidence that the form emission spectroscopy. However, in this case the bottom port oxidase is only periodically transporting electrons within of the microscope was fiber-optically
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