MECHANISMS OF LIPID-PEROXIDATION IN HUMAN BLOOD-PLASMA - A KINETIC APPROACH

There is strong evidence that the oxidation of plasma lipoproteins pla ys an important role in atherogenesis. The exact mechanisms by which l ipoprotein oxidation occurs in the presence of other plasma constituen ts, however, remains unclear. To investigate the role of different ant ioxidants for this process, we studied the oxidation of human plasma s upplemented in vitro with physiological amounts of major plasma antiox idants alpha-tocopherol, ubiquinol-10, ascorbate, urate, bilirubin and albumin. The plasma was diluted 2-fold and oxidized by 3.75 mM Cu(II) . The concentrations of the antioxidants, fatty acids, linoleic acid h ydroperoxides and oxycholesterols in oxidizing plasma were measured. T he oxidation was characterized by three consecutive phases similar to the known lag, propagation, and decomposition phases of low density li poprotein oxidation. The rate of the initiation of oxidation as calcul ated from antioxidant consumption rates was raised by supplementation with alpha-tocopherol or ascorbate. The oxidation rate in the lag phas e was lowered by supplementation with any of the antioxidants, whereas in the propagation phase the oxidation rate was slightly higher in su pplemented than in unsupplemented plasma. The kinetic chain length in the lag phase was less than one in supplemented plasma and about one i n unsupplemented plasma. The chain length in the propagation phase was between three and six for all plasma samples. A higher rate of urate consumption and a reduced rate of alpha-tocopherol consumption were fo und in plasma supplemented with ascorbate in comparison with unsupplem ented plasma. These data suggest that: (i) the reduction of Cu(II) by alpha-tocopherol and ascorbate is a major initiating event in Cu(II)-c atalyzed oxidation of human plasma; (ii) the following lag phase is ca used by radical-scavenging effects of all antioxidants with alpha-toco pherol as a major lipophilic and urate as a major hydrophilic scavenge r; (iii) interactions between antioxidants, such as regeneration of as corbate by urate and of alpha-tocopherol by ascorbate, take place duri ng the lag phase; (iv) in the absence of added antioxidants the oxidat ion in the lag phase can occur via a chain reaction; and (v) in the pr opagation phase the oxidation is not inhibited by antioxidants and occ urs autocatalytically. (C) 1997 Elsevier Science Ireland Ltd.