A non-linear regression program in BASIC for estimating Km and Vmax

Next step in 2nd generation glucose biosensors: Ferrocene-loaded electrospun nanofibers

Glucose determination is one of the most common analyses in clinical chemistry. Employing biosensors for this purpose has become the method of choice for home use for diabetic patients. To limit the impact of dissolved O₂ concentration or possible interferences (known hindrances in the classical glucose detection approach), a variety of mediated pathways have been explored. Herein, an ingenious, facile and low-cost approach for immobilization of redox mediator within nanofibrous mats is presented. This '2^nd generation' biosensor is able to avoid common issues such as leaching or diffusion barriers whilst providing the necessary close contact between the enzyme and the redox shuttle, for enhancing the detection accuracy and accelerate the response. Polyacrylonitrile nanofibers loaded with carbon nanotubes and ferrocene (PAN/Fc/MWCNT-COOH NFs) have been successfully prepared and applied as biosensing matrices upon cross-linking of glucose oxidase (GOD). The morphology of the NFs was investigated by means of scanning electron microscopy (SEM-EDX) and correlated to the kinetics of mediated electron transfer and to the efficiency in glucose detection, which were evaluated through cyclic voltammetry (CV) and amperometric measurements. The content of Fc was varied from 0.5 to 5.0 wt%, with optimum biosensor performance at 1.0 wt% exhibiting a linear range up to 8.0 × 10^-3 M with sensitivity of ~27.1 mAM^-1 cm^-2 and 4.0 μM LOD. Excellent stability (RSD 2.68%) during 40 consecutive measurements along with insignificant interference and accurate recovery in real sample analysis (~100%) make for a very reliable sensor that can easily render itself to miniaturization and has the potential for a wide range of practical applications.

Comparison of various estimation methods for the parameters of Michaelis-Menten equation based on in vitro elimination kinetic simulation data

The Michaelis-Menten equation is one of the best-known models describing the enzyme kinetics of in vitro drug elimination experiments, and takes a form of equation relating reaction rate (V) to the substrate concentration ([S]) via the maximum reaction rate (V_max) and the Michaelis constant (K_m). The current study was conducted to compare the accuracy and precision of the parameter estimates in the Michaelis-Menten equation from various estimation methods using simulated data. One thousand replicates of simulated [S] over serial time data were generated using the results of a previous study, incorporating additive or combined error models as a source of random variables in the Monte-Carlo simulation using R. From each replicate of simulated data, V_max and K_m were estimated by five different methods, including traditional linearization methods and nonlinear ones without linearization using NONMEM. The relative accuracy and precision of the estimated parameters were compared by the median values and their 90% confidence intervals. Overall, V_max and K_m estimation by nonlinear methods (NM) provided the most accurate and precise results from the tested 5 estimation methods. The superiority of parameter estimation by NM was even more evident in the simulated data incorporating the combined error model. The current simulation study suggests that NMs using a program such as NONMEM provide more reliable and accurate parameter estimates of the Michaelis-Menten equation than traditional linearization methods in in vitro drug elimination kinetic experiments.

Enhanced sensitivity of heart cells to adenosine and up-regulation of receptor number after treatment of guinea pigs with theophylline

Experiments were carried out in hearts from guinea pigs that were fed either the adenosine receptor antagonist theophylline (0.6 mg/ml) or no drug. The A1 adenosine receptor radioligand [125I]aminobenzyladenosine bound to a single affinity class of receptors in heart cell membranes from control animals with Bmax and KD of 18.3 +/- 1.0 fmol/mg protein and 3.7 +/- 0.6 nM, respectively (n = 8). Heart cell membranes from animals fed theophylline for 2, 7, and 14 days bound the radioligand with about the same affinity, but the number of binding sites was significantly increased (p less than 0.01) to 30.6 +/- 1.7 (n = 3), 30.0 +/- 0.8 (n = 3), and 27.3 +/- 2.9 (n = 4), respectively. Nearly identical results were obtained with membranes prepared from enzymatically dispersed ventricular myocytes. Fourteen days of theophylline treatment also produced a small increase (12%, p less than 0.01) in the number of binding sites in membranes derived from cerebral cortexes. Isolated ventricular myocytes prepared from animals fed no drug or theophylline for 7 days were used to determine the effect of adenosine on 20 nM isoproterenol-stimulated calcium current (ICa) measured by the whole-cell patch-clamp technique. Adenosine reduced isoproterenol-stimulated ICa without affecting the activation or inactivation kinetics of the current; ICa density was reduced less by 5 microM adenosine in cells from control (25 +/- 3 to 21 +/- 3 microA/microF) than in cells from theophylline-fed animals (26 +/- 5 to 17 +/- 2 microA/microF). Although a high concentration (0.5 mM) of adenosine abolished isoproterenol-stimulated ICa in cells from control or theophylline-fed animals, the IC50 for adenosine was sixfold less in cells derived from theophylline-fed animals than in cells from control animals (4.6 +/- 0.6 microM and 28.3 +/- 1.4 microM, respectively, p less than 0.01). In contrast, the increase in ICa in response to isoproterenol alone and the potency of acetylcholine to antagonize this effect of isoproterenol were the same in both groups of cells. A maximally effective concentration of R-phenylisopropyladenosine (0.1 mM) inhibited isoproterenol-stimulated cyclic AMP accumulation less in cardiomyocytes from control than from theophylline-fed animals (28.7 +/- 1.8% vs. 42.0 +/- 4.2%, p less than 0.05). In summary, exposure of the myocardium to theophylline increases the number of adenosine receptors and the effects of receptor occupancy by agonists. These findings imply that the endogenous concentration of adenosine is high enough in the normoxic guinea pig heart to chronically maintain adenosine receptors in a partially down-regulated state.