Stability of fluorescent-labeled rhIDU. Stability of the enzyme as a function of pH and storage temperature for labeled and unlabeled rhIDU are shown over the course of 70 days incubation. Labeled (solid lines) and unlabeled (dashed lines) enzymes were diluted to 16 μg/ml in 100 mM/50 mM phosphate-citrate buffer at pH 4.4 (A), pH 5.5 (B), and pH 7.4 (C) and these stock solutions were incubated at various temperatures, 4°C (blue), 25°C (green), 37°C (red) over a 70-day period. Enzyme aliquots were taken throughout this period of time and activity was estimated under the standard conditions (25 mM 4-MUI for 10 minutes at 22 °C). Units of activity are defined as nmoles of 4-MU product formed per hour.