Oxidation of albumin is enhanced in the presence of uremic toxins

Albumin has been considered a "sacrificial plasma antioxidant" due to the h igh reactivity of the protein sulfhydryl groups with oxidants such as hydro gen peroxide (H2O2) and hypochlorous acid (HOCl). Based on its large quanti ty and high turnover. It is considered as one of the most important plasma antioxidants for protecting key cellular and regulatory proteins. Since hem odialysis patients have lower overall levels of albumin and possible protei n modifications due to uremic toxins, we investigated whether modifications by various uremic toxins would affect the susceptibility of albumin to an oxidative challenge. We incubated bovine serum albumin in the presence of c arboxymethyllysine (CML) (10 mu mol/L-1 mmol/L), methyl glyoxal (50 mu mol/ L-5 mmol/L), p-cresol (100 mu mol/L-10 mmol/L) or hippuric acid (200 mu mol /L-20 mmol/L) for 16 hours at 37 degreesC and then subsequently added 0.5 m mol/L-1 mmol of H2O2/HOCl. We measured the extent of protein modification b y the loss of protein sulfhydryl groups, dityrosine formation and the forma tion of advanced oxidation protein products (AOPP}. Incubation of albumin w ith the uremic toxins caused a loss of protein sulfhydryl groups and an inc rease in dityrosines and AOPP. The presence of uremic toxins had no effect on the loss of protein. sulfhydryl groups after addition of H2O2/HOCl; howe ver, low levels of CML, p-cresol and methyl glyoxal inhibited the formation of AOPP and dityrosines. We suggest that uremic toxins may possibly play a role in mediating free radical initiated protein damage.