Low molecular weight (LMW) proteins include:
(1) retinol binding protein (RBP; 21,000 Daltons)
(2) kappa and lambda light chains
(3) beta-2 microglobulin (12,000 Daltons)
(4) alpha-1 microglobulin (30,000 Daltons)
(5) urine protein 1 (Clara Cell protein; 20,000 Daltons)
Low molecular weight proteinuria indicates excretion of one or more LMW proteins above the reference range (Norden et al). Tubular proteinuria is LMW proteinuria secondary to damage to the proximal tubules.
Excretion of larger proteins secondary to glomerular damage will mask the tubular proteinuria.
Measurement of LMW proteins in the urine needs to be done carefully because:
(1) the amount of these protein in the urine tends to be small in terms of weight
(2) common tests for proteinuria tend to measure larger proteins and are insensitive to LMW proteins
Urine protein electropheresis is one way of separating urine proteins that is readily available.
