Quality control material for activity determinations of urinary enzymes.

We examined the stability of N-acetyl-beta-D-glucosaminidase (EC 3.2.1.30), alanine aminopeptidase (EC 3.4.11.2), alkaline phosphatase (EC 3.1.3.1), gamma-glutamyltransferase (EC 2.3.2.2), and lysozyme (EC 3.2.1.17) in urine prepared by gel filtration and supplemented with albumin, or ethylene glycol, or ethylene glycol plus albumin during storage at -20 degrees C for a period of 12 months. The stability was assessed by linear regression analysis of monthly values versus time. All enzymes except for gamma-glutamyltransferase could be considered stable for about one year in all three control materials provided that maximum change of 10% of the starting enzyme activity is accepted as tolerable. If ethylene glycol is used as stabilizer, its suitability must be tested and its inhibitory effect on enzyme activities must be taken into account in intermethod comparisons, because in some methods, it may be removed in a pretreatment step.