Improved enzymatic assays for bile acids using resazurin and NADH oxidoreductase from Clostridium kluyveri

Survival Tests for Leptospira spp

Measuring viability is an important and necessary assessment in studying microorganisms. Several methods can be applied to Leptospira spp., each with advantages and inconveniencies. Here, we describe the traditional colony-forming unit method, together with two other methods based, respectively, on the reducing capacity of live cells (Alamar Blue^® Assay) and differential staining of live and dead cells (LIVE/DEAD BacLight^®). The Alamar Blue^® Assay uses the blue reagent resazurin, which can be reduced into the pink reagent resorufin by live cell oxidoreductases. Production of resorufin can be quantified by absorbance or fluorescence reading. The LIVE/DEAD BacLight^® assay uses a mixture of two nucleic acid dyes (Syto9 and propidium iodide) that differentially penetrate and stain nucleic acid of cells with decreased membrane integrity. The colony-forming unit method is labor-intensive but the most sensitive and linear method. The two other methods are not laborious and well-adapted to high-throughput studies, but the range of detection and linearity are limited.

Automated Droplet-Based Microfluidic Platform for Multiplexed Analysis of Biochemical Markers in Small Volumes

The ability to detect multiple analytes in a small sample volume has significance for numerous areas of research, including organs-on-chip, small animal experiments, and neonatology. The objective of this study was to develop an automated microfluidics platform for multiplexed detection of analytes in microliter sample volumes. This platform employed computer-controlled microvalves to create laminar co-flows of sample and assay reagent solutions. It also contained valve-regulated cross-junction for discretizing sample/reagent mixtures into water-in-oil droplets. Microfluidic automation allowed us to control parameters related to frequency of droplet generation and the number of droplets of the same composition, as well as the size of droplets. Each droplet represented an individual enzymatic assay carried out in a sub-nanoliter (0.8 nL) volume reactor. An enzymatic reaction involving target analyte and assay reagents produced colorimetric or fluorescent signals in droplets. Importantly, intensity of optical signal was proportional to the concentration of analyte in question. This microfluidic bioanalysis platform was used in conjunction with commercial "mix-detect" assays for glucose, total bile acids, and lactate dehydrogenase (LDH). After characterizing these assays individually, we demonstrated sensitive multiplexed detection of three analytes from as little as 3 μL. In fact, this volume was sufficient to generate multiple repeat droplets for each of the three biochemical assays as well as positive control droplets, confirming the quality of assay reagents and negative control droplets to help with background subtraction. One potential application for this microfluidic bioanalysis platform involves sampling cell-conditioned media in organ-on-chip devices. To highlight this application, hepatocyte spheroids were established in microfluidic devices, injured on-chip by exposure to lipotoxic agent (palmitate), and then connected to the bioanalysis module for daily monitoring of changes in cytotoxicity (LDH), energy metabolism (glucose), and liver function (total bile acids). Microfluidic in-droplet assays revealed increased levels of LDH as well as reduction in bile acid synthesis-results that were consistent with hepatic injury. Importantly, these experiments highlighted the fact that in-droplet assays were sufficiently sensitive to detect changes in functional output of a relatively small (∼100) number of hepatocyte spheroids cultured in a microfluidic device. Moving forward, we foresee increasing the multiplexing capability of this technology and applying this platform to other biological/medical scenarios where detection of multiple analytes from a small sample volume is desired.

Resazurin as an electron acceptor in glucose oxidase-catalyzed oxidation of glucose

The behavior of resazurin (1) as an electron acceptor in glucose oxidase (GOD)-catalyzed oxidation of glucose under anaerobic conditions is described. When a mixture of 1, glucose, and GOD in phosphate buffer (pH 7.4, 0.1 M) was incubated at 25 degrees C, the resulting solution turned purple to fluorescent pink due to the deoxygenated product, resorufin (2). On incubation of 1 with GOD alone or with H2O2 under essentially the same conditions, no color change was seen, indicating that generation of 2 in the enzymatic reaction is brought about through reduction of 1 by the reduced form (GODred) of GOD, which was also supported by the voltammetric behavior of 1. However, it was found that the enzymatic transformation of 1 to 2 is of no practical use as an indicator reaction for glucose determination using only GOD due to a slow reaction of 1 with GODred. Based on a ping-pong type mechanism with a steady-state approximation, KM and kcat for 1 as an electron acceptor from GODred were estimated to be 15+/-1.3 microM and (5.0+/-0.5) x 10(-2) s(-1), respectively.

Profiles of serum bile acids in liver diseases

Cholylglycine (CG) and sulfolithocholylglycine (SLCG) in fasting and postprandial serum were determined in patients with liver diseases by radioimmunoassay. In normal controls, fasting CG and SLCG were 54.67±68.66ng/100ml and 16.61 ±12.84ng/100ml respectively, while postprandial CG and SLCG were 61.21 ±37.29ng/100ml and 21.95± 15.9ng/100ml respectively. In liver disease, serum bile acids were elevated. The greatest increase was found in acute viral hepatitis but moderate or slight increase was also found in chronic active hepatitis, liver cirrhosis, and hepatoma. Fasting bile acids seem to be a sensitive index for hepatocellular dysfunction but not for differential diagnosis of liver diseases. In liver diseases except hepatoma, fasting CG and SLCG correlated significantly with total bilirubin, albumin, GOT, GPT, and alkaline phosphatase but not with cholesterol. Insignificant elevation of bile acids was found postprandially in patients with liver diseases as well as normal controls and postprandial bile acids were not more sensitive than fasting ones.

Effect of fenofibrate and LF 2151 on hepatic peroxisomes in hamsters

Hamsters were given a diet containing fenofibrate (0.5% or 0.05%) or its metabolite, LF 2151 (0.15% or 0.015%) or a standard diet for a 3-week period. At the end of this period, the analysis of plasma lipids showed that the mean plasma triglyceride concentrations were not significantly different in the five groups of animals. The mean plasma cholesterol concentrations were significantly reduced in animals treated with both drugs but only when given at the high dosage. No consistent changes were noted in the liver weight/body weight ratio and the DNA content of the liver; the number of peroxisomes was increased in the hepatocytes of animals given fenofibrate at the high dosage. Liver homogenates were fractionated and the fractions rich in peroxisomes were used for assays of several enzymes involved in lipid metabolism. Compared with the control animals, activity of cyanide-insensitive fatty acyl-CoA (FA-CoA) oxidizing system was significantly increased by fenofibrate at the high dosage, carnitine acetyltransferase activity was markedly increased by both drugs at the high dosage and catalase activity remained unmodified. As there was a significant inverse correlation between the peroxisomal activity of FA-CoA oxidizing system and the plasma cholesterol concentrations, it is suggested that the increase of peroxisomal beta-oxidation activity can be involved in the hypocholesterolemic action of fenofibrate and LF 2151. This is further substantiated by the finding that fenofibrate and LF 2151 were present in the peroxisomal fraction only in hamsters displaying hypocholesterolemia and high activity of FA-CoA oxidizing system. The presence of fenofibric acid in the plasma of hamsters given LF 2151 suggested that hepatocytes are able to generate the parent drug from this metabolite, underlining that the pharmacokinetics of fenofibrate are rather complex in hamsters.

Colorimetric enzymatic measurement of serum total 3 alpha-hydroxy bile acid concentrations without extraction

A new method for the simpler colorimetric assay of serum total 3 alpha-hydroxy bile acid concentrations without prior extraction has been investigated. The lowest concentration that could be reliably measured was 3 mumol/l and the results obtained were in agreement with those obtained using extraction and the spectrofluorimetric enzymatic assay (y = 1.007x - 1.86, r = 0.99, n = 36). Results for 20 normal sera collected after an overnight fast were 6.2 +/- 5.1 mumol/l (mean +/- SD) and in 20 normal plasma randomly collected were 4.9 +/- 4.2 mumol/l. This new assay is comparable in specificity and sensitivity to the older spectrofluorimetric enzymatic assays and convenient for routine use.

Determination of total bile acids in serum. A comparison of a radioimmunoassay with an enzymatic-fluorimetric method

A solid phase radioimmunoassay kit method for total conjugated bile acids has been compared to an enzymatic fluorimetric method for total serum bile acids. The methods were compared with respect to: precision, cross-reactivity (molar equivalence) of different bile salts, recovery of different bile salts from serum, the reference range for a healthy population, linearity, coefficient of correlation, diagnostic effectiveness, cost and ease of assay. Both assays seemed equally capable of predicting the presence or absence of liver disease. Radioimmunoassay had little advantage over the enzymatic-fluorimetric method. Its relative ease was far outweighed by its greater cost and poorer analytical performance.

Stoichiometry of the NADH-oxidoreductase reaction for dehydrogenase determinations

The NADH oxidoreductase reaction with resazurin was most rapid at pH 6.5. FMN (10 mumol/l) markedly stimulated the reaction, and the optimal concentration of resazurin was 50 mumol/l. The oxidation of NADH by NADH oxidoreductase with resaruzin as electron acceptor gave a variable yield of fluorescent product, resorufin. The yield was pH dependent and was greatest at pH 6.5. Measurements of oxygen consumption in the reaction mixture demonstrated that dissolved O2 was an alternative electron acceptor. The increased yield of resorufin at pH 6.5 was due to more rapid reduction of resazurin rather than oxygen. In contrast, at pH 9.0 oxygen was the preferred electron acceptor. The sensitivity of assays utilizing this indicator reaction can be improved by these optimized conditions.