Spectrofluorimeters are more complicated to handle and

there exist selleck products more sources for errors, therefore fluorimetric assays are unusual, and a deeper experience is needed (Cantor and Schimmel, 1980, Harris and Bashford, 1987, Guibault, 1990, Lakowicz, 1999 and Dewey, 1991). Similar arguments hold for CD and ORD measurements, which are valuable techniques for the observation of asymmetric compounds, like sugars (Cantor and Schimmel, 1980, Chance, 1991 and Adler et al., 1973). Enzymatic degradation of particles, like starch, can be observed by turbidimetry (Bock, 1980), while luminometry is applied for ATP dependent reactions (Campbell, 1989 and DeLuca and McElroy, 1978). Besides optical methods, electrochemical methods are in use, especially pH determinations for reactions proceeding with pH changes, like the liberation of acids by lipase or choline esterase. Since pH changes influence severely enzyme activity, a pH stat connected with an auto-burette is used, which keeps the pH constant by adding a neutralizing solution, its amount being Daporinad a direct measure of the proceeding reaction (Taylor,

1985). The methods mentioned so far allow the continuous, time-dependent following of the enzyme reaction (continuous assay). This is important for the determination of the reaction velocity and for evaluating the enzyme activity. Moreover, it permits the detection of erroneous influences and artifactual disturbances and especially the control of the reaction course (progress

curve). As will be discussed below, a catalysed reaction must initially follow a linear relationship, from which its velocity is derived. Due to depletion of substrates during the later progression the reaction slows down and finally ceases. Therefore it is important that for determination of the velocity only the linear part of the progress curve is taken, but if it is not possible to observe the complete progress curve, it cannot be confidently excluded, that calculation of the velocity includes also the non-linear part of the progress curve and aberrant results will be obtained ( Figure 3). This Idelalisib holds for all cases, where no direct signal for the conversion of substrate or product can be found. To determine the velocity the reaction must be stopped after a defined time and the amount of product formed or substrate converted must be analysed thereafter by a subsequent chemical indicator reaction or a separation method, like HPLC (stopped assay). Instead of a continuous progress curve these methods provide only one single point and the velocity must be calculated from the slope of a line connecting this point with the blank before starting the reaction. Such a procedure gives no guarantee that the measurement occurs indeed within the linear part of the progress curve and therefore control measurements at different reaction times must be undertaken to establish this fact.