How can storage time and temperature affect enzymic activities in urines?

Area of Science:

• Biochemistry
• Clinical Chemistry
• Urology

Background:

• Accurate enzyme measurements in urine are crucial for diagnosing various kidney and metabolic disorders.
• Understanding the stability of urinary enzymes under different storage conditions is essential for reliable diagnostic testing.
• Alanine aminopeptidase (AAP) and N-acetyl-beta-D-glucosaminidase (NAG) are commonly measured urinary enzymes.

Purpose of the Study:

• To investigate the impact of varying storage times and temperatures on the concentrations of urinary alanine aminopeptidase (AAP) and N-acetyl-beta-D-glucosaminidase (NAG).
• To determine optimal storage conditions for preserving the activity of these enzymes in urine samples.

Main Methods:

• Urine samples from 15 healthy individuals were analyzed.
• Enzyme activities of AAP and NAG were measured after storage under different conditions: room temperature, 4°C, -20°C, and -70°C for varying durations.
• The effect of sample centrifugation prior to storage was also assessed.

Main Results:

• Short-term storage (24 hours) at room temperature or 4°C did not significantly alter AAP and NAG levels.
• Both enzymes were well-preserved when stored long-term at -70°C.
• AAP activity showed a significant decrease after 1 month at -20°C, with levels dropping to approximately 8% of the initial value within 4 days.
• In contrast, NAG activity remained stable even after storage at -20°C for 1 month.
• Centrifugation was found to be non-critical for the measurement of these enzymes.

Conclusions:

• Storage temperature significantly impacts the stability of urinary AAP, with rapid degradation occurring at -20°C.
• Urinary NAG is highly stable across a range of storage temperatures, including -20°C.
• For accurate AAP measurements, samples should be stored at -70°C or processed quickly.
• NAG measurements are more robust to variations in storage conditions, making it a more stable biomarker in this regard.