Antioxidants and their Actions

Albumin, the major protein of human plasma (reference interval 535-760 / M), has the properties of copper 

Bilirubin (normal plasma concentration 20 iM) is able to scavenge singlet oxygen and peroxyl radicals. It has been proposed that bilirubin bound to human albumin contributes significantly to the non-enzymic antioxidant defences in human plasma (Stocker and Ames, 1987). 

It has been proposed that the a-tocopheroxyl radical can be recycled back to tocopherol by ascorbate producing the ascorbyl radical (Packer etal., 1979 Scarpa et al., 1984). The location of a-tocopherol, with its phytyl tail in the membrane parallel to the fatty acyl chains of the phospholipids and its phenolic hydroxyl group at the memisrane-water interface near the polar headgroups of the phospholipid bilayer, enables ascorbate to donate hydrogen atoms to the tocopheroxyl radical. The suitability for ascorbate and tocopherol as chain-breaking antioxidants is exemplified (Buettner, 

1993) in the feet that they are effective in relatively small amounts and their radical states are relatively unreactive, in either reducing or oxidizing capacities. Recent studies in experimental chemical systems have described studies in which the a-tocopheroxyl radical can act as a prooxidant (Bowry era/., 1992). This is perhaps not so surprising in view of the well-known characteristic of many antioxidants in their assumption of the characteristics of pro-oxidants at high concentration. 

Ubiquinol-10 (or coenzyme Qio see Corongiu et oL, 1993) (0.4-1.0/iM range of concentrations in human plasma) has recently been proposed as a chain-breaking antioxidant (Beyer, 1990) in LDLs as the first line of defence (Stocker et oL, 1991) and in liposomal systems. 

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