Mass spectrometric monitoring of albumin in uremia

Abstract

Background. Advanced glycation end products (AGEs) are a novel class of uremic toxins. In plasma, they are present in proteins and also in low molecular mass peptides. AGE-modified peptides are thought to bind and modify plasma proteins. Monitoring of the consequent increase in molecular mass of serum albumin may be used in surveillance of the clinical management of uremia. Methods. We investigated molecular mass changes of human serum albumin (HSA) glycated by glucose and methylglyoxal in vitro and of subjects with moderate renal impairment, end-stage renal disease (ESRD), ESRD on hemodialysis, and normal healthy controls by matrix-assisted laser desorption ionization mass spectrometry. Results. Fatty acid-free HSA had a molecular mass of 66,446 ± 114 D. Mean (±SD) molecular mass increases were HSA minimally glycated by glucose 399 ± 88 D (N = 5, P < 0.001), HSA highly glycated by glucose 6780 ± 122 D (N = 5, P < 0.001), HSA minimally glycated by methylglyoxal 73 ± 121 D (N = 5, P > 0.05), and HSA without fatty acid removal 535 ± 90 D (N = 5, P < 0.001). For HSA of human subjects, mean (± SD) molecular mass increases were normal healthy controls 243 ± 97 D (N = 5), moderate renal impairment 350 ± 83 D (P > 0.05 with respect to controls, N = 5), ESRD 498 ± 128 (P < 0.02 with respect to controls, N = 3), and ESRD on hemodialysis 438 ± 85 D (P < 0.02 with respect to controls, N = 5). The mean molecular mass of albumin of all groups was increased significantly with respect to that of fatty acid free albumin (P < 0.001). Conclusions. Only ESRD was associated with a significant increase in the molecular mass of HSA in vivo. Since this mass increase was very low and much lower than reported for AGE-modified peptides, it may reflect AGE formation on HSA by α-oxoaldehydes that accumulate in uremia, rather than modification of albumin by AGE-modified peptides. The molecular mass of HSA in vivo was indicative of a minimal and not a high extent of glycation.