[A study of microproteinuria in patients with collagen disease]

To examine the subclinical renal damage in collagen disease, we analyzed the excretion pattern of microproteinuria. We studied 58 collagen disease patients including 25 RA (rheumatoid arthritis) patients, 15 SLE (systemic lupus erythematosus), 5 PSS (progressive systemic sclerosis), 4 MCTD (mixed connective tissue disease), and 9 others. Urinary protein was not detected by urine dipsticks in all patients. Urinary proteins, which were concentrated to 5 mg/ml, were subjected to linear gradient (4-30%) SDS-PAGE and then transferred to nitrocellulose membrane by electrophoretic blotting method. The membrane was stained with Auro Dye and the blotted proteins were identified by enzyme immunoassay using specific antibodies. The percentages of albumin of whole urinary proteins were 27.2 +/- 13.7% in RA and 25.8 +/- 12.6% in PSS, which were significantly lower than that of controls (42.1 +/- 15.3%). However no significant difference in the percentage of urinary albumin was noted between SLE and controls. The percentages of low molecular weight (MW) proteins (proteins having smaller MW than albumin) were higher in RA and PSS. Especially the bands with MW of 25,200 were prominent and these percentages were 11.3 +/- 6.1% in RA and 14.6 +/- 9.1% in PSS, which were significantly higher than controls (5.1 +/- 3.5%). These bands with MW of 25,200 were demonstrated to be free light chains of immunoglobulins by western blotting method. From the above observations, protein excretion patterns in RA or PSS patients were so-called tubular proteinuria, and especially free light chain excretion was increased. We proposed that tubular dysfunction and abnormal production of free light chain might exist frequently in collagen diseases, especially RA and PSS.