N-ACETYL-?-D-GLUCOSAMINIDASE AS A BIOMARKER OF LEAD INDUCED RENAL TUBULAR DYSFUNCTION IN SPRAGUE-DAWLEY RATS

Abstract

There is epidemiological evidence that inorganic Pb may play a significant role in the development of chronic renal failure. However, the progression from acute Ph nephropathy to chronic nephropathy occurs over long periods of time and usually with continued exposure and is generally silent without any remarkable specific proteinuria. Consequently, many epidemiological and clinical studies failed to conclusively prove that prolonged exposure to Pb causes chronic nephropathy even though the clinical signs and histological evidence indicates that Pb may be an etiological factor.

Urinary NAG activity appears to be the only biomarker that responds at an early stage to Pb nephropathy. However, the close response/dose effect relationships are weak and there is much debate as to its significance.

Urinary NAG is a high molecular weight lysosomal enzyme found along the brush border of proximal tubules. It is stable and has high activity in urine. Currently, total urinary NAG and its isoenzyme profiles are mainly used for the diagnosis of renal diseases and renal function, more specifically, renal tubular function in a wide variety of clinical and occupational settings. However, there are numerous suggestions that NAG is too sensitive and non-specific. Thus, the urinary NAG isoenzymes are increasingly being used for the assessment of renal function as it has been demonstrated that different isoenzyme profiles were observed in different diseases. Urinary NAG B has been found to be more closely associated with renal damage than total urinary NAG.

The aim of this study is to assess the relationship between acute and chronic Pb exposure and the biomarkers of Pb exposure, in particular blood and kidney Pb concentrations, and urinary NAG excretion in a rat model to determine if total urinary NAG and its isoenzymes are suitable biomarkers of Pb induced nephropathy. A sensitive, reproducible and accurate automated method for the detection of urinary NAG isoenzymes in rats and humans was also developed.

NAG was found to be a suitable marker for the early detection of acute Pb induced renal dysfunction as it was well correlated to blood and renal Pb concentrations. The increase in urinary NAG excretion in acute exposure animals were not a physiological response to Pb as suggested by some reports, because proteinuria was observed following Pb administration. Increasing proteinuria was taken to be indicative of persistent renal tubular dysfunction as a result of Pb toxicity.

However, urinary NAG was found to be a poor marker for nephropathy due to chronic Pb exposure as its activity was only elevated after 3 months of exposure to Pb, by which other signs of nephropathy was apparent. The changes in the proportion of urinary NAG isoenzymes is a better marker of Pb nephropathy. Hence we found that the isoenzymc profile of NAG has the potential to be a powerful diagnostic tool for the assessment of renal dysfunction as a result of Pb nephropathy.

The HPLC method developed in this study for the separation of NAG isoenzymes in rats and humans was found to be accurate and reproducible. An in depth study of the rat isoenzymes may provide invaluable information on the role of NAG in the cell as well as elucidate the mechanisms for the increased excretion of NAG and the changes in NAG profile in response to renal injury.