Analysis of activity and inhibition of oxygen-dependent enzymes by optical respirometry on the LightCycler system

Combining the oxygen sensor based respirometry and 16S rRNA amplicon sequencing for the analysis of microbiota in commercial mince products

In this study, rapid respirometric microbial testing was combined with 16S rRNA amplicon sequencing, to assess the composition of microbiota in a total of 64 samples of commercial beef, turkey, lamb and pork mince. The O2 sensor-based respirometry system, while producing the anticipated total aerobic viable counts (TVC) data and patterns for most samples, also revealed unusual (linear) respiration profiles for some samples, mostly lamb and pork mince. The TVC values for beef mince, produced by respirometry and calculated using the available calibration equation, correlated well with the conventional plate counting method, ISO 4833-1:2013, 2013, while for the other species the correlation was less good. These effects, not observed in previous studies employing various food matrices, require further investigation. Using the same samples (crude homogenates) as in respirometry, the whole microbiome was also analysed by 16S rRNA amplicon sequencing for each mince-type. The sequencing showed an overall decrease in alpha diversity over shelf-life, with lamb and pork mince maintaining a proportion of rare taxa. Some taxa exhibited significant changes in abundance over shelf-life and after the respirometric analysis, with beef mince exhibiting a decrease in aerobic bacteria and an increase in facultative anaerobes. Beta diversity was also seen to depend on mince-type. Thus, the combined use of respirometry and sequencing techniques shows promise as a useful and unique analytical approach for food quality and safety evaluation, However, more data points and in-depth analysis are required to back up the findings of this initial study.

In Vivo Dynamic Monitoring of Small Molecules with Implantable Polymer-Dot Transducer

Small molecules participate extensively in various life processes. However, specific and sensitive detection of small molecules in a living system is highly challenging. Here, we describe in vivo real-time dynamic monitoring of small molecules by a luminescent polymer-dot oxygen transducer. The optical transducer combined with an oxygen-consuming enzyme can sensitively detect small-molecule substrates as the enzyme-catalyzed reaction depletes its internal oxygen reservoir in the presence of small molecules. We exemplify this detection strategy by using glucose-oxidase-functionalized polymer dots, yielding high selectivity, large dynamic range, and reversible glucose detection in cell and tissue environments. The transducer-enzyme assembly after subcutaneous implantation provides a strong luminescence signal that is transdermally detectable and continuously responsive to blood glucose fluctuations for up to 30 days. In view of a large library of oxygen-consuming enzymes, this strategy is promising for in vivo detection and quantitative determination of a variety of small molecules.

Multifunctional Phosphorescent Conjugated Polymer Dots for Hypoxia Imaging and Photodynamic Therapy of Cancer Cells

Molecular oxygen (O2) plays a key role in many physiological processes, and becomes a toxicant to kill cells when excited to 1O2. Intracellular O2 levels, or the degree of hypoxia, are always viewed as an indicator of cancers. Due to the highly efficient cancer therapy ability and low side effect, photodynamic therapy (PDT) becomes one of the most promising treatments for cancers. Herein, an early-stage diagnosis and therapy system is reported based on the phosphorescent conjugated polymer dots (Pdots) containing Pt(II) porphyrin as an oxygen-responsive phosphorescent group and 1O2 photosensitizer. Intracellular hypoxia detection has been investigated. Results show that cells treated with Pdots display longer lifetimes under hypoxic conditions, and time-resolved luminescence images exhibit a higher signal-to-noise ratio after gating off the short-lived background fluorescence. Quantification of O2 is realized by the ratiometric emission intensity of phosphorescence/fluorescence and the lifetime of phosphorescence. Additionally, the PDT efficiency of Pdots is estimated by flow cytometry, MTT cell viability assay, and in situ imaging of PDT induced cell death. Interestingly, Pdots exhibit a high PDT efficiency and would be promising in clinical applications.

Plasticity of insect body size in response to oxygen: integrating molecular and physiological mechanisms

The hypoxia-induced reduction of body size in Drosophila and Manduca is ideal for understanding the mechanisms of body size plasticity. The mechanisms of size regulation are well-studied in these species, and the molecular mechanisms of oxygen sensing are also well-characterized. What is missing is the connection between oxygen sensing and the mechanisms that regulate body size in standard conditions. Oxygen functions both as a substrate for metabolism to produce energy and as a signaling molecule that activates specific cellular signaling networks. Hypoxia affects metabolism in a passive, generalized manner. Hypoxia also induces the activation of targeted signaling pathways, which may mediate the reduction in body size, or alternatively, compensate for the metabolic perturbations and attenuate the reduction in size. These alternative hypotheses await testing. Both perspectives-metabolism and information-are necessary to understand how oxygen affects body size.

Extension and application of the "enzyme test bench" for oxygen consuming enzyme reactions

Within industrial process development, powerful screening techniques are required to select the optimal biocatalyst regarding such process characteristics as cost effectiveness, turnover number or space time yield. Conventional measurement of the initial enzyme activity, which is the established high throughput screening technique, disregards the long-term stability of an enzyme. A new model based technique called "enzyme test bench" was recently presented before by our group which addresses this issue. It combines the high throughput screening approach with an extensive enzyme characterization, focusing especially on the long-term stability. The technique is based on modeling enzyme activation and deactivation as temperature dependent reactions in accordance with the Arrhenius law. Controlling these reactions by tailor made temperature profiles, the slow long-term deactivation effects are accelerated and characterizing models are parameterized. Thus, the process properties of an enzyme can be predicted and included into the screening procedure. Moreover, the optimum process temperature as function of the envisaged operation time can be found by these means. In this work, the technique is extended to the important class of oxygen consuming reactions. For this aim, a suitable assay and a defined oxygen supply were established. This extended technique was applied to characterize and to optimize a complex, multi-stage laccase-mediator system (LMS). For the variation and optimization of the enzyme to mediator to substrate ratio, experiments in microtiter plates were performed. Predictions from this high throughput characterization were compared to long-term experiments in a RAMOS device (Respiration Activity Monitoring System), a technique for on-line monitoring of the oxygen transfer rate in shake flasks. Within the limits of the model validity, the enzyme test bench predictions are in good agreement with the long-term experiments.

Geranylgeraniol oxidase activity involved in oxidative formation of geranylgeranoic acid in human hepatoma cells

Geranylgeranoic acid (GGA), a 20-carbon acyclic polyprenoic acid (all-trans 3,7,11,15-tetramethyl- 2,4,6,10,14-hexadecatetraenoic acid) and its derivatives were developed as synthetic "acyclic retinoids" for cancer chemoprevention. Previously, we have shown the natural occurrence of GGA in various medicinal herbs and reported enzymatic formation of GGA from geranylgeraniol (GGOH) through geranylgeranial (GGal) by rat liver homogenates. Here, we present several lines of evidence that a putative GGOH oxidase is involved in GGA synthesis by human hepatoma cell lysates. First, conversion of GGOH to GGal did not require exogenous NAD(+), whereas the conversion from GGal to GGA absolutely required additional NAD(+). Second, GGal synthesis from GGOH was coupled with consumption of oxygen from the reaction mixture. Third, GGOH-dependent GGal synthesis activity was proteinase K-resistant and even enhanced by proteinase K treatment; GGOH oxidase activity was enriched in the mitochondrial fraction. Finally, recombinant human monoamine oxidase (MAO)-B, but not MAO-A catalyzed oxidation of GGOH to GGal. These data suggest that a putative mitochondrial GGOH oxidase is involved in the initial step of GGA synthesis from GGOH.

Optical probes and techniques for O2 measurement in live cells and tissue

In recent years, significant progress has been achieved in the sensing and imaging of molecular oxygen (O₂) in biological samples containing live cells and tissue. We review recent developments in the measurement of O₂ in such samples by optical means, particularly using the phosphorescence quenching technique. The main types of soluble O₂ sensors are assessed, including small molecule, supramolecular and particle-based structures used as extracellular or intracellular probes in conjunction with different detection modalities and measurement formats. For the different O₂ sensing systems, particular attention is paid to their merits and limitations, analytical performance, general convenience and applicability in specific biological applications. The latter include measurement of O₂ consumption rate, sample oxygenation, sensing of intracellular O₂, metabolic assessment of cells, and O₂ imaging of tissue, vasculature and individual cells. Altogether, this gives the potential user a comprehensive guide for the proper selection of the appropriate optical probe(s) and detection platform to suit their particular biological applications and measurement requirements.

Rapid and quantitative measurement of metabolic stability without chromatography or mass spectrometry

Metabolic stability measurements are a critical component of preclinical drug development. Available measurement strategies rely on chromatography and mass spectrometry, which are expensive and labor intensive. We have developed a general method to determine the metabolic stability of virtually any compound by quantifying cofactors in the mechanism of cytochrome P450 enzymes using fluorescence intensity measurements. While many previous studies have shown that simple measurements of cofactor depletion do not correlate with substrate conversion (i.e., metabolic stability) in P450 systems, the present work employs a reaction engineering approach to simplify the overall rate equation, thus allowing the accurate and quantitative determination of substrate depletion from simultaneous measurements of NADPH and oxygen depletion. This method combines the accuracy and generality of chromatography with the ease, throughput, and real-time capabilities of fluorescence.