The sequential addition steps specified under “Procedure” provide an internal control. The following observations were made:
After step (2), one should observe a very low fluorescence. If the fluorescence is too high readjust the settings of the fluorometer.
After step (3), the fluorescence intensity should immediately increase ca. 10-20-fold. If the fluorescence increase is very different, check the pH of the buffer.
After step (4), the fluorescence should not change by more than 10%. If it changes by a larger amount, check the lysine sample for impurities.
After step (5), the fluorescence should decrease first stepwise (immediately) and then more slowly with time. If the latter time-resolved fluorescence decay is not observed after 5 min, the enzyme may be inactive.
If the stepwise (immediate) decrease in fluorescence intensity in step (5) is very pronounced (we relate this to varying degrees of salts and impurities in the enzyme sample), the sequence of steps (4) and (5) can be exchanged to obtain visually improved enzyme kinetic traces. In this case the time-resolved fluorescence decay is only observed after the addition of substrate in step (5), while upon addition of enzyme in step (4) only an immediate stepwise decrease in fluorescence intensity is observed.