EFFECT OF TEMPERATURE AND PHASE-TRANSITION ON OXIDATION RESISTANCE OFLOW-DENSITY-LIPOPROTEIN

The study of the effect of temperature on the kinetics of low density lipoprotein (LDL) oxidation was carried out by measuring the conjugate d diene (CD) versus time curves at a fixed LDL concentration (0.1 mu M ) and at different Cu2+ concentrations (0.5-10 mu M) in a wide tempera ture range, from 10 degrees C to 45 degrees C. The core melting point of the LDL determined with differential scanning calorimetry was 31.1 degrees C. We have demonstrated that temperature exerts a clear effect in the Cu2+-meditated LDL oxidation, with a strong decrease in lag ti me and a notable increase in the rate of propagation. This temperature dependence of lag time and rate of propagation fully obeys the Arrhen ius law, suggesting that the core melting point of the LDL has no or o nly a minor effect on these oxidation indices. The Arrhenius plots of the binding of Cu2+ to LDL, measured by K, gave two breaks suggesting that this value is affected by the core transition of the LDL as well as by structural changes at around 15 degrees C. The mean activation e nergy during rate of initiation was 13.5 kcal/mol and tended to decrea se with increasing Cu2+ concentration. The activation energy in the pr opagation phase was 10.6 kcal/mol and was independent of Cu2+ concentr ation. In this work we have also shown that the CD method can be condu cted with high reproducibility and that a sucrose-supplemented plasma frozen at -80 degrees C can be used as a source of LDL with an unvaryi ng vitamin E content and reproducible oxidation properties.