Results of our conclusive study on urinary enzyme stability during sample storage are reported. We measured alanine aminopeptidase (AAP) and N-acetyl-β-D-glucosaminidase (NAG) in morning urines from 9 healthy normal subjects immediately after collection and throughout a 1-year storage at -70 and -20 °C. AAP proved to be quite stable at -70 °C (99.2% of the basal value at the end of the year). NAG is partially preserved (84.1 % of the basal value) at -70 °C, but significantly decreased (50.4%) at -20 °C.

We measured three renal tubular brush-border enzymes (lactate dehydrogenase, LDH, EC 1.1.1.27; gamma-glutamyltransferase, GGT, EC 2.3.2.2; and alkaline phosphatase, AP, EC 3.1.3.1) in morning urine samples from 48 healthy subjects to check whether different storage times and temperatures could modify enzyme concentrations. Short-term (24 h) storage time at room temperature or 4 degree C does not affect urinary enzyme activity. A few days of freezing, at -20 or -70 degrees C, dramatically lowers LDH and AP values; GGT is partially preserved only at -70 degrees C, if the sample has been previously centrifuged. Urinary enzymes investigated in this study are extremely labile at low temperatures.

We measured a tubular brush-border enzyme (alanine aminopeptidase, AAP) and a lysosomal hydrolase (N-acetyl-β-ö-glucosaminidase, NAG) in morning urines from 15 healthy normal subjects to check if different storage times and temperatures could modify enzyme concentrations. Short-term (24 h) storage time at room temperature or 4 ° C does not affect AAP and NAG activities. Both enzymes are well preserved at -70 °C. AAP dramatically falls after 1 month at - 200 C, lowering to about 8 % of the initial value after only 4 days of storage. On the contrary, NAG is well preserved at these storage conditions. Centrifugation has revealed not critical for measurement of these two enzymes.

A sensitive and precise automated assay of urinary lactate dehydrogenase (EC 1.1.1.27), alkaline phosphatase (EC 3.1.3.1) and γ-glutamyltransferase (EC 2.3.2.2) is described. For this purpose, we used a BM/Hitachi System 704 model and reagents for automated analysis of serum enzymes from Boehringer Mannheim. However, the schedules of enzyme chemistry parameters recorded by the autoanalyzer and the spectrophotometric calibration are reprogrammed to meet requirements deriving from urine adoption and to optimize the enzyme assay in this unusual medium.