To evaluate tubular damage in diabetic patients, we measured the 24 h urinary excretion of five enzymes (N-acetyl-β-D-glucosaminidase, γ-glutamyl transpeptidase, dipeptidyl aminopeptidase IV, alanine aminopeptidase and alkaline phosphatase) that originate in renal proximal tubular cells. Studies were performed on 118 non-insulin-dependent diabetic patients, 59 non-diabetic patients with chronic renal disease and 47 normal control subjects. First, the correlation between renal function, glycaemic control and urinary enzyme excretion was investigated. Secondly, the subjects were treated by controlled diet therapy to assess the effects of better glycaemic control on urinary enzyme excretion. Regardless of a diabetic or non-diabetic cause of renal dysfunction, all of the five enzymes showed abnormal urinary excretion in patients with renal insufficiency (serum creatinine concentration > 2.0 mg/dl). In diabetic patients, however, an increase in N-acetyl-β-D-glucosaminidase excretion and a decrease in γ-glutamyl transpeptidase excretion were noted even in those who had no signs of renal dysfunction, including microalbuminuria. Moreover, the excretion of these two enzymes had a higher degree of correlation with glycaemic control and renal function than did that of the other three enzymes. Multiple regression analysis revealed that excretion of N-acetyl-β-D-glucosaminidase is best correlated with urinary protein (r2 = 0.35), whereas excretion of γ-glutamyl anspeptidase is closely associated with glomerular filtration rate (r2 = 0.33). In diabetic patients, diet therapy improved glycaemic control but had no effects on renal function, microalbumin excretion and β2-microglobulin excretion. In contrast, abnormal N-acetyl-β-D-glucosaminidase and γ-glutamyl transpeptidase excretion were both significantly improved in diabetic patients without overt renal dysfunction. However, in nondiabetic patients and diabetic patients with renal insufficiency, there were no changes in urinary enzyme excretion. These results suggest that there is reversible tubular damage in the early stages of diabetic nephropathy. Measurement of urinary excretion of N-acetyl-β-D-glucosaminidase and γ-glutamyl transpeptidase may be especially useful in the detection of this tubular dysfunction.
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