N-ACETYLNEURAMINIC ACIDS (NANA) - A POTENTIAL KEY IN RENAL CALCULOGENESIS

N-Acetylneuraminic acids (NANA) promote binding of calcium ions to mac romolecules and cells, increase the intrinsic viscosity of glycoprotei ns and facilitate gel formation in water. Since these properties are c rucial in urinary calculogenesis, we evaluated NANA levels in urine an d serum as well as their expression in kidney tissues. Using a modifie d thiobarbituric acid assay, the evaluation of free and bound NANA in 24-h urine samples revealed a ratio of 1.87 in 33 non-stone-formers bu t a reversed ratio of 0.84 in 41 recurrent calcium oxalate stone-forme rs. Time kinetics revealed a gradual rise in NANA expression until 48 h of culture and a significantly higher release into supernatants of p apillary renal epithelial cells (REC) when compared with cortical REC. To examine NANA distribution in kidney tissues, paraffin-embedded bio psies from five normal and six stone-forming kidneys were labeled with the biotinylated NANA-specific lectins Maackia amurensis (MAA) and Sa mbucus nigra (SNA). Immunohistochemistry revealed intense luminal MAA reactivity of distal tubular REC and collecting ducts in 96.7% and 91. 5% of normal and stone-forming kidneys respectively. By contrast, ther e was a marked difference between normal and stone-forming kidneys for SNA reactivity (17.7% vs 95%) at the same locations. Finally, the gly cocalyx of recurrent stone-formers showed altered sialylglycoside link ages [alpha(2,6) instead of alpha(2,3)] that may indicate an altered R EC function. Given the calcium-binding potential of NANA, their increa sed local concentration within the glycocalyx layer in the distal neph ron may either initiate stone formation or facilitate attachment of mi crocrystals to REC.