Significance of glycosaminoglycans for the formation of calcium oxalate stones

Glycosaminoglycans (GAG) are polysaccharide chains composed of repeating disaccharides of identical composition. Little is known about the mechanism of their excretion, but there is no doubt that urinary GAGs are degradation products of high molecular weight proteoglycans. Renal excretion takes place chiefly as glomerular filtration, and tubular reabsorption or secretion has not been demonstrated. Differences in the literature comparing GAG excretion in urolithiasis patients and healthy subjects are mainly attributable to methods of analysis and noncomparability of the investigation conditions. We found no differences between the two groups in several series. It is interesting to note that GAG excretion in men is significantly higher than in women, that a circadian rhythm of GAG concentration and excretion occurs in healthy subjects on a standardized diet, and that values are raised postprandially and at night. Seasonal course of GAG excretion curves is almost synchronous for men and women, irrespective of the absolute values, and GAG excretion in the spring and summer significantly exceeds that in winter months by up to 50%. All crystallization models cited demonstrate that GAG reduce the risk of calcium oxalate stone formation. Inhibitors of crystal growth and aggregation act by blocking the growth sites. Inhibition of calcium oxalate crystallization is also attributed to direct binding of calcium to GAG. In the presence of urate ions, and favorable pH, the ability of chondroitin sulfate C to bind calcium may be impaired by as much as 31%. These measurements support the concept that urate ions interact with GAG in urine.(ABSTRACT TRUNCATED AT 250 WORDS)