Abstract
In patients with diabetes mellitus, increased oxidative stress may contribute to low density lipoprotein (LDL) oxidation and elevated levels of circulating products of inflammation. A randomised, placebo-controlled study was conducted in patients with type 2 diabetes mellitus to compare the effect of four weeks of supplementation with tomato juice (500 ml/day), alpha-tocopherol (800 IU/day) or vitamin C (500 mg/day) on LDL oxidation, circulating levels of C-reactive protein (C-RP), soluble vascular cell adhesion molecule-1 (sVCAM-1) and intercellular adhesion molecule-1 (sICAM-1), and circulating products of lipid peroxidation. Plasma lycopene levels increased nearly three fold with the consumption of tomato juice and LDL resistance (lag time) to copper ion stimulated oxidation increased by 42% (P=0.001). The magnitude of the change with tomato juice was comparable to the corresponding increase during supplementation with a pharmacological dose of alpha-tocopherol (54%, P=0.001). Alpha-tocopherol also decreased plasma C-RP (-49%, P=0.004) and increased plasma cholesterol concentration (9%, P=0.022). Circulating levels of adhesion molecules, erythrocyte TBARS, plasma indices of lipid peroxidation and plasma glucose did not change significantly during the study. These findings indicate drinking tomato juice is an effective way to increase plasma lycopene levels and the intrinsic resistance of LDL to oxidation in diabetic patients. Alpha-tocopherol increases the resistance of LDL to oxidation and reduces systemic inflammatory activity but these potentially anti-atherogenic actions are opposed by a concomitant increase in plasma cholesterol levels.
The second study in this thesis focused on the development of a model which may reflect LDL oxidation in the arterial intima more closely than the frequently used copper ion induced oxidation of isolated LDL. Low density lipoproteins can bind to proteoglycans rich in heparin and chondroitin sulphate in the arterial intima and consequently may become a target for atherogenic modification by myeloperoxidase (MPO). Experiments were conducted to examine the susceptibility to peroxidase/hydrogen peroxide (H20 2) catalysed oxidation of resolubilised LDL, that has been precipitated from serum with heparin and from native LDL with heparin, chondroitin sulphate, dextran sulphate, and polyethyleneglycol. In addition, the effects of antioxidants and components of human serum on the oxidation of heparin-LDL in a peroxidase/ H20 2 system were investigated. The LDL from complexes with glycosaminoglycans and dextran sulphate were oxidised rapidly by horse radish peroxidase (HRP) and H202 (mean t1/2max for conjugated diene formation of 3-5 minutes) while there was little oxidation of native LDL and polyethyleneglycol-LDL during the 30 minute incubation period. Aggregated LDL was not oxidised by HRP/H202. The formation of thiobarbituric acid reacting substances (TBARS) paralleled the change in conjugated dienes during oxidation of heparin-LDL. Heparin-LDL was also more rapidly oxidised than native LDL by MPO/H202. Oxidation of heparin-LDL by peroxidases did not require free tyrosine and was almost totally inhibited by butylated hydroxytoluene (BHT) and ascorbate, and was unaffected by vitamin E or urate. Increasing concentrations (0-14.9%) of betalipoprotein deficient serum (BLPDS) significantly (P<0.0001) inhibited the formation of TBARS during heparin-LDL oxidation catalysed by HRP and MPO. The inhibitory activity was removed by dialysis and gel-filtration of BLPDS and was not restored by addition of physiological levels of ascorbate, tyrosine and reduced thiols (cysteine) to gel-filtered BLPDS. These results indicate that LDL can form complexes with glycosaminoglycans rendering them particularly susceptible to oxidation by peroxidases. Furthermore LDL oxidation may be inhibited by small, water soluble compounds in the human serum but not by vitamin E. These findings may be relevant to the formation of oxidatively modified LDL in the artery wall.