Mass spectrometry of urinary β2 microglobulin oligomer in patients with proteinuria

Urinary β2 microglobulin (β2-m) is a marker of renal tubule dysfunction; however, β2-m might become degraded under acidic conditions. To confirm the degradation and consequent deactivation of β2-m under acidic conditions, we used matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) to detect the levels and forms of β2-m in the urine samples of patients with high proteinuria (n = 21) and healthy subjects (n = 6). β2-m was purified in crude form using immunoprecipitation. A signal of 11.74 kDa, corresponding to the molecular weight of β2-m, was detected in all samples. In addition, several high-molecular-weight proteins were detected in a patient as integrals of the intensity at 11.74 kDa. These results indicate that posttranslational modifications of β2-m might be involved in the pathological process of proteinuria. Therefore, MS can be used for monitoring proteinuria and predicting the risk of progression.