Stabilities of N-acetyl-beta-D-glucosaminidase (NAG) isoenzymes in urine: advantage of NAG isoenzyme B measurement in clinical applications

N-Acetyl-beta-D-glucosaminidase (NAG) is a widely used urinary enzyme for t he assessment of renal diseases. We studied the stabilities of NAG isoenzym es in urine at 37 degrees C by enzyme assay and ELISA using a model simulat ing in vivo conditions. The stabilities were found to be affected by the pH . Under mild acidic condition (about pH 6), there was no significant loss o f enzymatic activity of NAG isoenzyme A, enzymatic activity of NAG isoenzym e B and immunological activity of NAG isoenzyme B even after 8 h incubation . In contrast, under alkaline condition (about pH 8), the enzymatic activit y of NAG isoenzyme A was rapidly lost, whereas both enzymatic and immunolog ical activities of NAG isoenzyme B were maintained at more than 80% of thei r initial values. Also, we found that the ratios of endogenous NAG isoenzym e B to total NAG were elevated in alkaline urine samples. These results ind icate that NAG isoenzyme A, which is a major isoenzyme in normal urine (pH 5-7), seems to be inactivated in alkaline urine. Our results suggest that f or alkaline urine, NAG isoenzyme B should be measured to avoid misinterpret ation of total NAG enzymatic activities. (C) 1998 Published by Elsevier Sci ence B.V. All rights reserved.