Contaminations, soiling or air bubbles can produce such effects and may be eliminated by careful manipulation; otherwise the assay system should be changed. In principle any chemical reaction, and thus also any enzyme reaction, is reversible, and may be observed both from the substrate as well EPZ015666 cell line as from the product side. However, reactions releasing energy (exergonic reactions, e.g. cleavage reactions) strongly favour one direction (quasi-irreversible reactions), while energy-consuming (endergonic) reactions are grossly disfavoured. Consequently,

enzyme assays use normally the favoured direction. Enzyme reactions that do not show a strictly favoured direction (reversible reactions) like dehydrogenases or isomerases can be tested from both sides. Usually the direction easier to achieve will be preferred, e.g. better stability and availability of substrates as well as instrumental aspects. An important advantage of quasi-irreversible reactions is the fact that the substrate will be completely converted to product, while reversible reactions convert the substrate to product only until the equilibrium is reached, Atezolizumab at the end

of the reaction both substrate and product remain in the assay solution in a constant ratio. For example, the equilibrium for the isomerase reaction between glucose to fructose is nearly at 50%, and thus at the end of the reaction both sugars will be present in comparable concentrations, irrespective of whether the reaction started from glucose or from fructose as substrate Carbohydrate (Antrim et al., 1979 and Lehmacher and Bisswanger,

1990). The alcohol dehydrogenase reaction with ethanol and NAD as substrates is more convenient than the back reaction with the toxic and volatile acetaldehyde and the expensive and less stable NADH. Moreover it is easier to observe a reaction starting from zero with an increasing absorption, instead to start with the high absorbing NADH. Unfortunately, the equilibrium favours the back reaction. However, with a trick the reaction can be forced in the desired direction, trapping the released protons at high pH and the acetaldehyde by a subsequent reaction with semicarbazide (Bergmeyer, 1983). For enzyme assays complete conversion of the substrate to product is preferred. Analysis of the product is easier in the absence of substrate and also the linear initial velocity is longer. Difficult detectable enzyme reactions are frequently coupled with easily observable reactions, preferentially NAD(P)H dependent dehydrogenases. An example is the hexokinase reaction (1) connected with the glucose-6-phosphate dehydrogenase (2): equation(1) Glucose+ATP→glucose-5-phosphate+ADPGlucose+ATP→glucose-5-phosphate+ADP equation(2) Glucose-6-phosphate++NADP→gluconate-6-phosphate+NADPH++HGlucose-6-phosphate+NADP+→gluconate-6-phosphate+NADPH+H+ The second, the indicator reaction can easily be detected by the absorption increase at 340 nm.

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