Measurements of Human Plasma Oxidation (2006)
AuthorsOsborn, Annashow all
The oxidation of lipids and antioxidants has been extensively studied in human plasma but little attention has been given to how plasma proteins are oxidised. Proteins make up the majority of biomolecules in cells and plasma and therefore are the most likely reactants with oxidants and free radicals. Previous studies in the laboratory had shown that peroxyl radicals generated by the thermolytic decay of 2-azobis (2-amdinopropane) dihydrochloride (AAPH) generated significant amounts of protein hydroperoxides, but only after a six hour lag period. In this study the existence of the six hour lag period was confirmed and shown by dialysis of the plasma to be due to the presence of low molecular weight antioxidants. The addition of both uric acid and ascorbic acid to the dialysed plasma restored the lag phase suggesting that in vivo these antioxidants act to prevent protein hydroperoxide formation. Lipid oxidation was also observed in the plasma but only after a two hour lag phase. This was the first time lipid oxidation has been observed in the absence of protein oxidation. The lipid lag phase was also abolished by dialysis of the plasma and restored by the addition of ascorbic acid and uric acid. The kinetics of tocopherol loss suggests that the tocopherol radicals act to inhibit lipid oxidation by transferring the electrons to the water-soluble ascorbate. The loss of ascorbate appears to cause the formation of a tocopherol radical mediate the lipid peroxidation process. Overall the data shows ascorbic acid scavenging the peroxyl radicals while uric acid acts to reduce the overall AAPH generated radical flux. In a separate investigation, the production of protein-bound DOPA (PB-DOPA) on albumin during X-ray radiolysis and copper mediate Fenton oxidation was investigated using a fluorescence based derivatisation method (ED-DOPA), which was compared with the more specific acid hydrolysis and HPLC analysis method. The ED-DOPA method consistently gave a much higher reading that the HPLC based methods, suggesting that the ED-DOPA method was measuring DOPA plus DOPA oxidation products. This was confirmed by oxidising X-ray radiolysis generated PB-DOPA with Cu++ to cause DOPA oxidation. The Cu++ treatment drastically increased the level of signal given by the ED-DOPA assay while HPLC analysis showed all the DOPA had been oxidised.