Open Access-MiPNet18.10 O2k vs multiwell

Mitochondrial Physiology Network 18.10(04): 1-8 (2014)
2013-2014 OROBOROS
Version 4: 2014-06-12
Oxygraph-2k and
system: specifications
for respirometry and
OXPHOS analysis
Gnaiger E, Fasching M
high-resolution respirometry
Schöpfstr. 18, A-6020 Innsbruck, Austria
[email protected];
1. Mitochondrial and cell respiration
High-resolution respirometry (HRR) combines long-term expertise in
instrumental design, software development and O2k-Protocols developed
for mitochondrial physiology and pathology. These set unique qualitative
and quantitative standards summarized as the O2k-Concept extended to
O2k-Fluorometry and the O2k-MultiSensor system.
combined to a Power-O2k for high-resolution with high output.
"High resolution designs (i.e., O2k, OROBOROS Instruments) maximize
respirometric sensitivity and precision (minimal O2 leak and highly sensitive
electrodes), reducing the biological sample size required. Software advances in
flux derivations of changes in chamber PO2 also permit real-time reporting of
respiratory kinetics (Datlab, OROBOROS Instruments), which improves data
analyses over other systems requiring visual assessments of steady-state
kinetics" Perry CG, Kane DA, Lanza IR, Neufer PD (2013) Methods for assessing
mitochondrial function in diabetes. Diabetes 62: 1041-1053.
“Without compromise on HRR features, the O2k provides robustness and
reliability of routine instrumental performance. To increase throughput
particularly in research with cell cultures and biopsy samples, the user-friendly
integrated concept with full software support (DatLab) makes it possible to apply
several instruments in parallel, each O2k with two independent chambers” Pesta
D, Gnaiger E (2012) High-resolution respirometry. OXPHOS protocols for human
cells and permeabilized fibres from small biopisies of human muscle. Methods
Mol Biol 810: 25-58.
[email protected]
MiPNet18.10 O2k vs multiwell
2. O2k versus multiwell respirometer
No single design is best for all. A specific respirometric instrument,
therefore, cannot cover all applications in the best way. In this regard, the
OROBOROS Oxygraph-2k for high-resolution respirometry and multiwell
respirometers for high throughput are complementary. Below, the O2k or
Power-O2k and the 24- or 96-well XFe (Seahorse Bioscience) are
compared with regards to specifications and applications.
A. Oxygraph-2k
The state-of-the art respirometer for quantitative high-resolution
respirometry and comprehensive OXPHOS analysis with extension
of respirometry [pmol O2∙s-1∙mg-1 or pmol O2∙s-1∙10-6 cells] by modules
for simultaneous real-time monitoring of ROS production, mt-membrane
potential, ATP production, Ca2+, NO or acidification rate by O2kFluorometry or potentiometry.
B. Multiwell
Multiwell systems are designed for high-throughput screening. Results
are semi-quantitative, when merely relative changes are obtained.
Methodological limitations are apparent when respiration is reported in
terms of pmol O2/min. How many cells were in the chamber?
3. Are specifications comparable?
A. Oxygraph-2k
The specifications of the OROBOROS O2k include several sole-source
instrumental features:
 Critical selection of materials yielding nearly diffusion-tight chambers.
 Long-term stability and linearity of the polarographic oxygen sensor
 Highly automatic and fully documented calibration routines and
instrumental background correction.
 Electronically controlled thermal environment in the range of 4 °C to 47
°C (2 °C at lower ambient temperature) with high temperature stability
(better than ±0.002 °C operated at room temperature).
 The limit of detection of oxygen flux is ±1 pmol O2∙s-1∙ml-1 in the
normoxic range. The limit of detection of oxygen concentration is 5
nmol/l (0.005 µM) with bracketing zero oxygen calibrations.
Specifications are available as open-information:
B. Multiwell
In some multiwell systems no specifications are given on sensitivity
(detection limit of oxygen flux; lower detection limit of oxygen
concentration; non-linearity and restricted linear range). Which well-towell reproducibility is guaranteed (see also temperature control)?
OROBOROS Oxygraph-2k
MiPNet18.10 O2k vs multiwell
4. Accuracy of chamber volume and mixing
A. Oxygraph-2k
The O2k-chamber has a standard volume of 2.0 ml and is accurately
calibrated (better than ±1% at an error of <20 µl, depending on
calibrated pipettes). The effective chamber volume (excluding the
injection capillary) is stirred rigorously to maintain a homogenous system.
B. Multiwell
No information is provided on the accuracy of the chamber volume in a
multiwell system (7-10 µl for the XF24; Perry et al 2013). This inaccuracy
translates directly to errors in the calculation of oxygen flux in the closed
chamber. Similarly, accurate final concentrations of titrated substances
are not known. Mixing by moving the sensor/injector part up and down a
few times may be inadequate. Undefined diffusion layers develop during a
measuring cycle.
5. Glass vs plastic
A. Oxygraph-2k
The O2k-Chambers are made of Duran glass
and are closed by PVDF or PEEK stoppers which
are as diffusion tight as titanium stoppers. The
magnetic stirrer bars are coated by PVDF or
PEEK. Teflon with its high oxygen solubility is
avoided (Gnaiger 1995). Viton O-rings are used
for sealing the stoppers. Butyl rubber gaskets
Duran glass O2k-Chamber
provide the seals for the oxygen sensors. These
sealing materials minimize oxygen diffusion into or out of the
experimental chambers.
The O2k not only minimizes the effect of oxygen backdiffusion by
avoiding inappropriate plastic materials, but additionally implements
automatic correction for instrumental background flux. Standardized
protocols (SOPs) are available to evaluate and improve the accuracy of
instrumental background correction. These instrumental tests can be
performed automatically using the Titration-Injection microPump (TIP2k)
with standard setups for feedback-control by the DatLab software.
B. Multiwell
Oxygen storage in the plastic materials of multiwell plates leads to high
oxygen backdiffusion. Since the problems are well known (Gnaiger 1995),
specifications should be provided on oxygen backdiffusion. Test protocols
should be applied for evaluation of such specifications (Gnaiger 2008).
At the high surface-to-volume ratio in a small well, the problem of
using plastic materials is not restricted to oxygen diffusion. Lipid soluble
substances (uncouplers, inhibitors) partition between the aqueous and
plastic phases, so that the surface-attached biological sample is exposed
to undefined effective concentrations.
OROBOROS Oxygraph-2k
MiPNet18.10 O2k vs multiwell
6. Quantification of amount of sample: cell
number, mitochondrial protein, tissue mass
A. Oxygraph-2k
In experiments with isolated mitochondria, tissue homogenates or
suspended intact or permeabilized cells, the final concentration in the
O2k-chamber is either defined by the preparation of the added
suspension, and/or determined by taking a quantitative subsample from
the chamber. In this way, the measured oxygen flux (per volume) can be
expressed accurately per unit of biological sample (per mg protein, per
million cells, etc) [pmol O2∙s-1∙mg-1 or pmol O2∙s-1∙10-6 cells].
In experiments with permeabilized muscle fibers or other tissues, the
tissue mass is determined before adding the sample into the O2k-chamber
(e.g. 0.7 mg wet weight of mouse heart, 2 mg wet weight of human
skeletal muscle). Oxygen flux can then be expressed per tissue mass
(mass-specific flux, reflecting mitochondrial density and functional mtquality).
The flexibility of the DatLab-software allows real-time display of
respiratory flux per unit sample (per mg, or per million cells) or per
volume of the aqueous medium.
B. Multiwell
How many cells are actually enclosed in the compartment for
measurement of respiration in a well? Which fraction of isolated
mitochondria or cells is outside versus inside the effective chamber? How
can the recorded change in oxygen concentration be converted into
respiration per million cells or per mg protein? Without solving these
problems, no quantitative measurements of respiration are possible.
Results reported as pmol O2/min lack meaning.
7. Flexibility: MultiSensor versus multiwell
A. Oxygraph-2k
The modular concept of the O2k-MultiSensor system
The O2k is designed as a flexible
modular system. The O2k-Core supports
add-on O2k-Modules for simultaneous
measurement of
oxygen flux
additional fluorometric measurement of
ROS production, mt-membrane potential
(TMRM, safranin), Ca2+, ATP-production
or potentiometric measurement of mtmembrane potential with TPP+ or TPMP+
(ion sensitive electrode, ISE), saponin
(using the same ISE) or pH. The DatLab
software provides full flexibility for O2kMultiSensor monitoring.
OROBOROS Oxygraph-2k
MiPNet18.10 O2k vs multiwell
B. Multiwell
The XFe is restricted to the additional measurement of pH. Specifications
should be given on sensitivity [pH] and measurement of acidification rate
[µpH/s]. Extracellular acidification rate is not to be confused with a
quantitative measurement of glycolysis. Oxygen flux of 50 pmol∙s-1∙ml-1
corresponds - at an assumed O2 flux to extracellular H+ flux ratio of 1:1 to a pH change of about 86 µpH/s in a very weak buffer (2 mM). What is
the drift of the pH signal?
8. OXPHOS analysis with multiple substrateuncoupler-inhibitor titrations
Substrate-uncoupler-inhibitor titration (SUIT) protocols
Range [h:min]: 1:30
diagnostic tests of mitochondrial respiratory function to study the complex
interactions of coupling and substrate control in a single assay, thus
increasing the information obtained per unit sample and per unit time.
More than 20 titration steps may be included in a single SUIT protocol.
The Figure illustrates a SUIT protocol with NIH3T3 fibroblasts (0.24∙106
cells/ml) for measurement of routine respiration in intact cells, followed by
permeabilization by digitonin, CI linked LEAK respiration and OXPHOS
capacity (glutamate+malate, GM; and ADP), convergent CI+II electron
input (succinate, S), cytochrome c test, inhibition by oligomycin and
uncoupler titration, CII-linked respiration (rotenone, Rot) and further (not
shown) inhibition of CIII by Antimycin A (Gnaiger 2012 MitoPathways).
O2 Flow per cells (F) [pmol/(s*Mill)]
O2 Concentration (F) [nmol/ml]
A. Oxygraph-2k
B. Multiwell
The number of titrations into a well is limited to a maximum of four. The
XFe, therefore, is not suited for application of SUIT protocols and OXPHOS
analysis. In this respect, the multiwell approach yields low throughput,
since many wells are required for multiple titrations, and high inter-well
variability represents a confounding factor.
9. Tissue preparations and cells
A. Oxygraph-2k
All mitochondrial preparations including permeabilized cells or muscle
fibres, homogenates and isolated mitochondria can be used for studies
OROBOROS Oxygraph-2k
MiPNet18.10 O2k vs multiwell
performed with the O2k. Intact or permeabilized suspended blood cells
and suspension cultures including yeast are ideally suited for the O2k.
Monolayer cell cultures are trypsinized and studied in suspension.
Neuronal cells may be studied attached to a disk inserted into the O2k
(Jones and Brewer 2009).
Intact C. elegans is a perfect model for the O2k, whereas more
delicate living animals, such as zooplankton, are likely to be put under
improper stress in the stirred O2k-chamber.
B. Multiwell
Cells cultured in monolayer in the wells are the superior model for the
"Use of permeabilized muscle fiber bundles has not been validated in
the XF Extracellular Flux Analyzer" (Perry CG, Kane DA, Lanza IR,
Neufer PD (2013) Methods for assessing mitochondrial function in
diabetes. Diabetes 62: 1041-1053).
Permeabilized muscle fibres are seriously oxygen limited at oxygen levels
at and below air saturation without stirring. Permeabilized cells may not
remain attached to the wall and therefore impose a problem for the XFe
technology, similar to tissue homogenate and isolated mitochondria.
Stirring permeabilized cells and tissues in homogenous suspension is
desirable but not possible with XFe technology.
10. Oxygen and temperature control
A. Oxygraph-2k
The oxygen regime can be controlled in routine applications of the O2k for
respiratory studies of hypoxia and hyperoxia. Oxygen kinetics of
mitochondrial respiration is made possible by resolution of oxygen
concentration in the nanomolar range and minimum oxygen backdiffusion.
Experimental temperature is controlled at unique stability of ±0.002
°C in the range of 2 °C to 47 °C. As a control, temperature and Peltier
power are continuously measured and can be displayed any time.
B. Multiwell
Experimental temperature cannot be regulated below room
temperature. Temperature stability and homogeneity between wells are a
critical issue without being monitored, potentially resulting in a systemic
well-to-well bias.
Control of the oxygen regime is restricted in routine applications to
intermittent equilibration of the unstirred medium with atmospheric
oxygen and declining oxygen levels during measurement. Measurements
at low oxygen levels are not possible due to high oxygen backdiffusion,
resulting in problems with zero oxygen calibration. The limit of detection is
not specified. Incubation in gas controlled bench chambers is required for
hypoxic or hyperoxic measurements.
OROBOROS Oxygraph-2k
MiPNet18.10 O2k vs multiwell
11. Quality versus quantity
A. Oxygraph-2k
The OROBOROS Oxygraph-2k for high-resolution respirometry (HRR)
sets the gold standard for highly accurate quantitative measurements
(which is high quality), following a scientific strategy. Comprehensive
OXPHOS analysis has been successfully introduced by SUIT protocols now
widely applied with the O2k (Gnaiger 2012 MitoPathways). High quality of
instruments and methods is required in research and clinical applications.
O2k-MultiSensor modules, particularly O2k-Fluorescence, extend HRR
beyond respirometry, making the O2k the most accurate and versatile
instrument for cellular and mitochondrial physiology and bioenergetics.
Bioenergetics made simple?
Scientific methods are developed and applied to help understanding
cell metabolism. Opening new ways to a better understanding of cell
metabolism requires a scientific enthusiasm and devotion to hard work
beyond the easy ways of superficial plug-and-play approaches.
Commercial organizations advertise the XFe as making cell metabolism
even easier. Companies may assist scientists instrumentally and
methodologically, but cannot make the subject of cell metabolism more
easy. Oxygen and pH: is this really cell metabolism revealed? Integration
of catabolism and anabolism, ATP levels and ATP turnover, cell membrane
and mt-membrane potentials, redox states and intermediary metabolite
levels, control of metabolic pathways - this and more is cell metabolism
way beyond oxygen and pH (Gnaiger 2012 MitoPathways).
B. Multiwell
With only four titrations per well with the XFe (i) OXPHOS analysis is
restricted to the simplest protocols with limited information, and (ii) large
numbers of separate runs are necessary for evaluation of optimum
uncoupler concentrations or saturating substrate concentrations.
12. Running costs and financial issues
A. Oxygraph-2k
The running costs for the O2k are very low, as experienced worldwide by
>500 O2k-users and many enthusiastic MiPNet Reference Laboratories.
 OroboPOS membranes: A membrane replacement is not required
over periods of several months. The costs for new membranes and
electrolyte, therefore, are less than € 10 per year.
 Media: Calculate 3 ml per run per chamber, e.g. MiR05 or MiR06.
 Chemicals: Substrates, uncouplers, inhibitors, specific effectors.
 Washing: With deionized or distilled water, pure ethanol (removing
inhibitors) and 70% ethanol (antimicrobial storage).
Based on long-term experience, annual running costs are significantly less
than € 1,000 for O2k-spares (e.g. sealing rings, spare sensor, spare glass
OROBOROS Oxygraph-2k
MiPNet18.10 O2k vs multiwell
chamber). In O2k-MultiSensor applications, spare sensors (e.g. glass pH
electrode) may add € 700 to € 1,400 running costs per year.
Power-O2k - a 'best'
investment: With two
the OROBOROS O2k is
several O2k instruments
can be obtained at the
cost of an XFe. Multiple
O2k-Chambers provide a
unique Power-O2k HRR system for quantitative O2k measurements at low
running costs for high output.
B. Multiwell
The running costs are extremely high, based on expensive
dischargeable cartridges for single use only. How many of the wells of a
dischargeable plate can actually be used for independent measurements?
Several wells are required for calibration. Edge effects may eliminate the
use of wells on the sides. If more than four consecutive titrations are
required, more wells have to be allocated for a single functional assay.
Elaborating a protocol for starting an experimental series requires a large
number of test runs, so that the cost of discharged wells in an entire
experiment approaches the investment in a second O2k.
The primary investment costs of the XFe system are exceedingly high
when compared with the Oxygraph-2k, particularly considering the limited
scope of the XFe technology (limitation of titrations, limitations of
MultiSensor extensions, limitation on quantification of results).
The running costs of the O2k are by far more economic than the high
running costs of the XFe. The XFe running costs calculated over a single
year cover the investment in a new O2k-Core including its running costs.
MiPArt by Odra Noel
OROBOROS Oxygraph-2k