Charge selectivity and urine amylase isoenzymes

The urinary excretions of salivary and pancreatic amylase were studied in 718 type I diabetic patients and 51 control subjects, as part of a multicenter study on diabetic nephropathy in 15 Spanish hospitals. It was found that the urinary ratio of salivary to pancreatic amylase (S/P ratio), that in normal subjects is always below 1, was elevated in 35.4% of diabetic patients, whereas microalbuminuria was present in 19.8%. The prevalence of elevated S/P ratio was also higher than that of microalbuminuria at the first years from the onset of the disease, but the prevalence of microalbuminuria was higher in patients with a long duration of the disease. alpha 1-microglobulin and microalbuminuria paralleled their prevalences during the disease, when measured in a group of patients. Overnight urine samples were obtained on three consecutive weeks from the diabetic patients, and a nested ANOVA analysis showed that the intra-individual variation of the urine parameters measured (albumin, salivary and pancreatic amylase, and beta-NAG) was very small and not statistically significant. All these findings suggest that in type I diabetes mellitus, loss of negative charges of GBM would induce preferential excretion of the anionic salivary amylase over the more cationic pancreatic amylase, and that this phenomenon is more frequent and appears earlier than microalbuminuria. The mechanisms for the increased excretion of salivary amylase and albumin into urine seem to be at least partly different. On the contrary, increase in urinary excretion of albumin and alpha 1-microglobulin in these patients are correlated, suggesting a tubular participation in the mechanisms of production of microalbuminuria.