The reaction time for stopped assays is usually indicated in the protocol and it must be assumed that this time is indeed within the range of the initial velocity. One must, however, be aware that any modification of the protocol, like higher enzyme activities, reduced substrate concentrations or change of the assay temperature, can cause the stop PI3K inhibitor time to fall outside

the permitted range. In such cases the linear progression of the reaction should be checked by performing several assays varying the stop time. Any enzyme assay requires a blank. For stopped assays the blank value is obligatory to determine the velocity from the difference between the stopped value and the blank, while with continuous assays the velocity is calculated from the slope of progress curve. This can be done without a blank value, but even here a blank is needed to adjust the instrument to zero, otherwise the reaction may fall

Sirolimus mouse outside the observation range of the system. Usually the assay mixture without the starting component is taken as blank, but care must be taken that the starting component does not change the blank. Otherwise another component must be taken to initiate the reaction. When the signal of the substrate is higher than that of the product, as is the case for dehydrogenase reactions with NADH as substrate, the signal will decline into the negative area. This is no principal problem, but if the system is adjusted to zero before starting, the reaction will run out of the observation range. In such cases the instrument should be adjusted to a higher value before starting, or the assay mixture without the substrate should be taken as a blank. It must be established that the blank remains constant during the measuring period. Sometimes, however, the blank show a considerable drift, which may influence the reaction 4-Aminobutyrate aminotransferase course, and thus the result of the assay. Often the drift progresses in a constant linear (positive or negative) manner. Such drift may be caused by the instability of the instrument, e.g. warming

up of photometric lamps and a longer accommodation time for the instrument will eliminate the problem. But also spontaneous side reactions, oxidative processes, instability of a component, incipient turbidity or other processes in the assay mixture can be responsible for the drift. In such cases its origin should be identified and as far as possibly eliminated, because such reactions will change the assay mixture, especially if it is kept for a longer time during an extensive test series. If the origin of the disturbance cannot be eliminated, the drift must be considered for the calculation of the enzyme velocity. Supposing the effect to be constant and reproducible under defined conditions, the velocity can be corrected by a constant drift value.