Variability in alpha-tocopherol antioxidant activity in the core and surface layers of low- and high-density lipoproteins

The effect of alpha-tocopherol enrichment of low- and high-density lipoproteins on Cu(2+)-catalyzed lipid peroxidation in the hydrophobic core and in the hydrophilic envelope of lipoproteins was investigated by using two pyrene derivatives, namely, cholesteryl pyrenyl hexanoate (P6Chol) and pyrene dodecanoyl sulfatide (P12CS). The progressive decrease in fluorescence of P6Chol was used to monitor lipid peroxidation in the core of LDL and HDL, whereas that of P12CS was used to follow lipid peroxidation in the envelope of both lipoproteins. alpha-Tocopherol enrichment of LDL and HDL was obtained by incubating blood plasma at 37 degrees C with different concentrations of the vitamin (25-500 microM) before lipoprotein separation. The incorporation of alpha-tocopherol in LDL and HDL presents a progressive, time-dependent increase up to 200 microM alpha-tocopherol, then a plateau up to 500 microM. In the envelopes, the added tocopherol causes a great decrease in the rate of peroxidation and a dramatic increase in the latency phase in both lipoproteins. In the cores the lengthening of latency phase resulting from alpha-tocopherol enrichment was by far greater in LDL than in HDL, and the decrease in the rate of peroxidation in both lipoproteins was less than in the envelopes.