Asymptomatic low molecular weight proteinuria: studies in five patients

Asymptomatic low molecular weight proteinuria, a disease recently reported by Suzuki et al. [1985], was found in five boys, two pairs of brothers and a sporadic patient aged 3 to 11 years. Their urinary proteins contained 56% to 67% of proteins of less than 40,000 mol wt, defined as low molecular weight proteins by Suzuki et al. [1985], an indication that proximal tubular reabsorption of these proteins is impaired in these patients. Their glomerular function tests and intravenous urography were normal. An attempt was made to identify urinary low molecular weight proteins in these patients, using Western blotting analysis of the protein bands separated by sodium dodecylsulfate polyacrylamide gel electrophoresis. All five proteins tested were detected: alpha 1-acid glycoprotein (mol wt 44,000), alpha 1-microglobulin (mol wt 33,000), retinol-binding protein (mol wt 21,000), lysozyme (mol wt 14,000), and beta 2-microglobulin (mol wt 11,800). The latter two proteins had been identified in the disease by other means by Suzuki et al. [1985], while the other three were newly identified. Light microscopic studies of renal biopsy specimens from these patients revealed in three of four patients tested focal global or segmental glomerular sclerosis with scattered intratubular casts and focal tubular atrophy. Immunofluorescence staining of the renal biopsy specimens for the five proteins revealed some in the lumens of the proximal tubules and in the casts in the distal or collecting tubules, while only retinol-binding protein was found in the epithelial cytoplasm of the proximal tubules.