Background: The proximal kidney tubule contains a large number of lysosomes involved in the breakdown of intracellular as well as reabsorbed proteins. When tubular protein reabsorption is overburdened or when there is tubular cell damage, higher urinary excretion of lysosomal enzymes, e.g. cathepsins and N-acetyl-beta-D-glucosaminidase (NAG), can be found. We compared urinary cathepsin B (CB) activity with NAG in diabetic patients.
Material/methods: Using fluorogenic substrates, urinary and plasma CB and NAG activities in 130 type 2 diabetic patients with varying stages of albuminuria and 42 control subjects were determined. Early morning urine samples were used.
Results: In the patients, only higher values of plasma NAG were found. In urine, CB and NAG activities increased progressively from normoalbuminuria, through microalbuminuria to macroalbuminuria group. The normoalbuminuria group had both enzyme activities higher than healthy controls. Urine CB activity in the patients also increased gradually to tertiles of urinary NAG. Only urinary CB activity was significantly associated with glycemic state. The correlation was stronger in the patients with poor glycemic control. The plasma/urine ratios for both CB and NAG decreased in the patients compared with controls.
Conclusions: Determination of urinary CB activity might be useful as a non-invasive surrogate marker of incipient nephropathy.