Urinary trehalase as an early indicator of cadmium-induced renal tubular damage in rabbit

Renal toxicity of heavy metals (cadmium and mercury) and their amelioration with ascorbic acid in rabbits

Cadmium and mercury are among the most toxic and dangerous environmental pollutants that may cause fatal implications. Vitamin C is an important chain-breaking antioxidant and enzyme co-factor against heavy metals. The objective of the present study was to evaluate the toxicological effects of cadmium chloride, mercuric chloride, and their co-administration on biochemical parameters of blood serum and metal bioaccumulation in kidneys and also to elucidate the protective effect of vitamin C in rabbits against these metals. In the current research, cadmium chloride (1.5 mg/kg), mercuric chloride(1.2 mg/kg), and vitamin C (150 mg/kg of body weight) were orally administered to eight treatment groups of the rabbits (1, control; 2, vitamin; 3, CdCl₂; 4, HgCl₂; 5, vitamin + CdCl₂; 6, vitamin + HgCl₂; 7, CdCl₂ + HgCl₂, and 8, vitamin + CdCl₂ + HgCl₂). After the biometric measurements of all experimental rabbits, biochemical parameters viz. creatinine, cystatin C, uric acid, and alkaline phosphatase (ALP) and metal bioaccumulation were determined using commercially available kits and atomic absorption spectrophotometer, respectively. The levels of creatinine (28.3 ± 1.1 μmol/l), cystatin C (1932.5 ± 38.5 ηg/ml), uric acid (4.8 ± 0.1 mg/day), and ALP (51.6 ± 1.1 IU/l) were significantly (P < 0.05) increased due to administration of mercuric chloride but in the presence of vitamin C, the effects of mercuric chloride on creatinine (21.9 ± 1.4 μmol/l), cystatin C (1676.2 ± 42.2 ηg/ml), uric acid (3.9 ± 0.1 mg/day), and ALP (43.3 ± 0.8 IU/l) were less as compared to metal-exposed specimens. Similar results were found in rabbits treated with cadmium chloride and vitamin C and also with co-administration of both metals and vitamin C. Because of the bio-accumulative nature of cadmium chloride and mercuric chloride, these metals were accumulated in kidneys of rabbits, which might lead to deleterious effects. The results of the present study provide an insight into the toxicity of the cadmium chloride, mercuric chloride, and/or their combination on biochemical parameters as well as kidneys of the rabbits and the ameliorating potential of vitamin C against these metals is also evaluated.

ELISA for urinary trehalase with monoclonal antibodies: a technique for assessment of renal tubular damage

BACKGROUND

alpha,alpha-Trehalase, located on renal proximal tubules, is a glycoprotein that hydrolyses alpha,alpha-trehalose to two glucose molecules. Urinary trehalase reflects damage to renal proximal tubules, but its activity has not been measured routinely because measurement of catalytic activity is rather complicated and because conventional assays for enzyme activity might not reflect all of the trehalase protein because of enzyme inactivation in urinary samples.

METHODS

We established novel monoclonal antibodies for human trehalase and a sandwich ELISA for quantification of urinary trehalase. We determined the urinary trehalase protein concentration with this ELISA and trehalase catalytic activity, and the results of these two methods were compared.

RESULTS

The ELISA system was more sensitive than the detection of enzyme activity and could detect a subtle difference in the amount of trehalase present in renal diseases. The within- and between-assay CVs in the ELISA were 6.7-7.6% and 6.2-8.2%, respectively. Highly significant increases in both the quantity and activity were seen in patients with nephrotic syndrome (acute phase), Lowe syndrome, and Dent disease. The quantities were 70- to 200-fold greater, whereas enzyme activities were, at most, 10-fold higher than those of control subjects. In the detection of small amounts of trehalase in patients with chronic glomerulonephritis and renal anomalies, quantities were better than enzyme activities.

CONCLUSIONS

We have established an ELISA system for quantification of urinary trehalase that uses novel monoclonal antibodies. Our ELISA system is simpler and more sensitive than a conventional activity assay and reflects trehalase protein. This ELISA can be a useful as a common tool for clinical assessment of renal proximal tubular damage.

Molecular cloning, sequencing and expression of cDNA encoding human trehalase

A complete cDNA clone encoding human trehalase, a glycoprotein of brush-border membranes, has been isolated from a human kidney library. The cDNA encodes a protein of 583 amino acids with a calculated molecular weight of 66,595. Human enzyme contains a typical cleavable signal peptide at amino terminus, five potential glycosylation sites, and a hydrophobic region at carboxyl terminus where the protein is anchored to plasma membranes via glycosylphosphatidylinositol. The deduced amino acid sequence of the human enzyme showed similarity to sequences of the enzyme from rabbit, silk worm, Tenebrio molitor, Escherichia coli and yeast. Northern blots revealed that human trehalase mRNA of approx. 2.0 kb was found mainly in the kidney, liver and small intestine. Expression of the recombinant trehalase in E. coli provided a high level of the enzyme activity. The isolation and expression of cDNA for human trehalase should facilitate studies of the structure of the gene, as well as a basis for a better understanding of the catalytic mechanism.

Cadmium inhibition of L-alanine transport into renal brush border membrane vesicles isolated from the winter flounder (Pseudopleuronectes americanus)

Using isolated brush border membrane vesicles from the kidney of the winter flounder (Pseudopleuronectes americanus), we have studied the effect of cadmium on L-alanine transport. Pretreatment of vesicles with 0.1 mM Cd2+ resulted in inhibition of L-alanine uptake in the presence of a NaCl (but not KCl) gradient. Inhibition was due to a specific interaction with the sodium-alanine cotransport system and not a change in the driving forces for alanine transport, since Cd2+ did not affect sodium-dependent D-glucose uptake. The effect of Cd2+ on Na+-alanine cotransport showed mixed-type inhibition which is only partially reversible by EDTA. Cd2+ uptake itself was shown to be time and temperature dependent, resulting in binding to both sides of the membrane. No direct correlation was possible between inhibition of L-alanine transport and the amount of Cd2+ taken up by the membranes. Nevertheless, the striking time dependence of the effect of Cd2+ on sodium-dependent L-alanine uptake and the inability of EDTA to reverse the inhibitory action of Cd2+ suggest that Cd2+ inhibits Na+-alanine cotransport at the cytoplasmic side of the membrane.

Urinary trehalase activity as an indicator of kidney injury due to environmental cadmium exposure

One hundred and seventy-eight subjects, patients with Itai-itai disease and their family members, aged 12-87 years living in a cadmium (Cd)-polluted area in the Jinzu River basin (Cd-exposed group) and 176 controls (control group) were examined. In the Cd-exposed group urinary trehalase increased with increasing age, urinary beta 2-microglobulin (beta 2-m) and retinol-binding protein. Although urinary cadmium was higher in the Cd-exposed group, no particular correlation was found between urinary trehalase and urinary cadmium. Seventeen men and 11 women showed raised urinary trehalase activities despite normal values of urinary beta 2-m (less than 300 micrograms/g.creatinine), suggesting that urinary trehalase increases earlier than urinary beta 2-m. In 19 patients with Itai-itai disease included in the Cd-exposed group, urinary trehalase decreased with decreasing reciprocal of serum creatinine, suggesting that urinary trehalase decreases in the most advanced cases of chronic cadmium nephropathy due to reduced tubular cell mass.

The mechanism of cadmium-induced lysozyme enhancement in rabbit kidney

The effect of cadmium on the renal lysozyme level was examined by injecting male albino rabbits subcutaneously with 1 mg cadmium/kg body weight three times a week for 1 or 3 months. The lysozyme level in the renal brush border membrane of the cadmium-treated animals was elevated ten-fold. The lysozyme activity in the liver and small intestine tissue homogenates of rabbits was elevated by a 1-month treatment with cadmium, markedly elevated in the kidney, but markedly reduced in the spleen and lungs. Exposure to cadmium for 3 months produced an essentially similar effect on the enzyme level in the tissue, except for the lungs in which the lysozyme level returned to the preinjection level. This marked increase in the lysozyme level in the kidney of cadmium-treated rabbits was confirmed by an indirect immunofluorescent antibody technique. In control animals, intracellular distribution of the enzyme was selectively distributed to only a small number of proximal tubules, with none distributed in the medulla or glomerulus. However, after expose to cadmium, the renal tubules showed strongly positive lysozyme staining. In addition to an increase in intensity of the specific fluorescence, this enzyme was widely distributed not only in the proximal convoluted portion, but also in the straight portion of the proximal tubules, which essentially showed no enzyme activity under normal conditions. The enzyme in these cells was evenly distributed throughout the cytoplasm. The plasma lysozyme level increased immediately after the administration of cadmium, and detectable amounts of the enzyme began to appear in urine from the 3rd week after the first injection, with a 1-week lag after the maximum level of lysozyme in the plasma. This high level of plasma lysozyme, varied two-to four-fold over the control, and lysozymuria continued throughout the experiment. The concentration of cadmium in the renal cortex was 141 micrograms/g wet tissue at 1 month, and 208 micrograms at 3 months. In conclusion, the cadmium-induced enhancement of the lysozyme level in the renal cortex may be due primarily to the elevation of the lysozyme level in plasma by cadmium. The enzymatic high net positive charge, characteristic of lysozyme, may contribute greatly to this mechanism. In addition, the excretion of a large amount of lysozyme into the urine observed in a later stage may be due to the concomitant occurrence of leakage from the destroyed tubular cells and reduced tubular reabsorption of filtered enzyme, whereas lysozymuria at an early stage may be solely due to excess amounts of plasma lysozyme.