The effect of various oestrogens and progestogens on the susceptibility of low density lipoproteins to oxidation in vitro

Objective: To investigate the effect of different oestrogens and progestogens, at various' concentrations, on the oxidation of low density lipoproteins (LDL) in vitro. Method's: Oestradiol, oestrone, oestriol and equilin, as well as medroxyprogesterone acetate, norgestrel and norethisterone, were added to isolated male LDL, before it was oxidised in he presence of copper ions at 37 degrees C. The oxidation process was monitored spectrophotometrically by the production of conjugated dienes. The lag time to oxidation and the maximum rate of propagation of the reaction were used as measures of the resistance and susceptibility of the LDL to oxidation respectively. Results: The lag time was increased from 43.7 +/- 1.5 min (mean +/- SEM) for LDL without any added hormone, to 81.2 +/- 1.0 min by 1 mu M oestradiol (P < 0.01), 77.9 +/- 4.6 min by 1 mu M oestrone (P < 0.01), 67.6 +/- 6.2 min by 1 mu M equilin (P < 0.01), and 51.8 +/- 2.8 min by 1 mu M oestriol (P < 0.05). The maximum rate of propagation of the reaction was decreased From 0.23 +/- 0.01 nmol conjugated dienes/mg LDL-protein/min (mean +/- SEM) (control LDL) to 0.14 +/- 0.006 nmol/mg/min by oestradiol (P < 0.01), 0.15 +/- 0.00Bnmol/mg/min by oestrone (P < 0.01), 0.17 +/- 0.012 nmol/mg/min by equilin (P < 0.01) and 0.19 +/- 0.014 nmol/mg/min (P < 0.05) by oestriol. The progestogens alone had no antioxidant effect, nor did their addition to the oestrogens influence their antioxidant activity. Conclusions: These results demonstrate that all oestrogens investigated have an inhibitory effect on LDL oxidation in vitro. The magnitude of this effect varied, being of the order oestradiol > oestrone > equilin > oestriol.