Effect of supplementation of smoking men with plain or slow release ascorbic acid on lipoprotein oxidation.

OBJECTIVE: To study the effects of two month ascorbic acid supplementation on in vitro lipoprotein oxidation resistance and on in vivo lipid peroxidation, and to compare the absorption of two ascorbic acid preparations. DESIGN: Randomized, single blinded and placebo-controlled clinical trial. SETTING: Men, aged 36-65 y, smoking 11-40 cigarettes daily. SUBJECTS: Sixty-two subjects were recruited by newspaper advertisements and randomized. Fifty-nine subjects completed the study. INTERVENTION: Subjects were randomized into three groups to receive 250 mg BID of plain or slow release ascorbic acid tablets or placebo daily for two months. In the pharmacokinetic part of the study, the absorption of the ascorbic acid preparations was followed for 12 h. MAIN OUTCOME MEASURES: Plasma malondialdehyde (MDA) concentration and the oxidation resistance of VLDL + LDL. For the pharmacokinetic study, the area under the plasma concentration curve (AUC) of ascorbic acid. RESULTS: Plasma reduced ascorbic acid increased by 32% in the plain ascorbate group and by 54% in the slow release group during a two month supplementation. Plasma MDA increased in the plain ascorbic acid group compared with placebo group (P < 0.05), but there were no significant differences in the changes in lipoprotein oxidation reactions induced by copper or by hemin and H2O2. Plasma reduced and total ascorbic acid AUC values were significantly higher in both plain and slow release ascorbate groups compared with placebo group. CONCLUSIONS: Oral supplementation of 500 mg of ascorbic acid daily for two months alone without any other antioxidant does not appear to have protective effect on either in vitro lipoprotein oxidation resistance or in vivo lipid peroxidation in smoking men, but might even promote the formation of MDA.

Inhibition of microsomal glucose 6-phosphatase by unsaturated aliphatic aldehydes and ketones.

Aldehydes and ketones with one double bond conjugated to the carbonyl group inhibited the enzyme glucose 6-phosphatase, which is embedded in the microsomal membrane. The Michaelis constant, Km and the maximal rate of reaction, V, were affected in a way dependent on the inhibitor's chain-length: trans-2-pentenal and 1-penten-3-one increased Km linearly with concentration and had almost no effect on V, whereas trans-2-nonenal caused a large increase in V but only a small and non-linear change in Km. The effect of the short-chain aldehydes on the kinetic parameters increased with chain-length, but pentenone increased Km more than did trans-2-heptenal and conjugated dienals did not act as inhibitors. Therefore, sterical effects apparently are of importance. Washing the microsomes after incubation with hexenal or heptenal did not substantially decrease the inhibition, but with nonenal the inhibition was reduced by washing. Inhibition by the SH-group blocking reagent p-hydroxymercuribenzoate was competitive to inhibition by the alkenals. It is concluded that the alpha-beta unsaturated oxo-compounds inhibit glucose 6-phosphatase by binding covalently to an important mercapto group and that perturbation of the enzyme's membrane environment also plays a part in the inhibition.