Effect of alpha-tocopherol on LDL oxidation and glycation: in vitro and in vivo studies

D Li,S Devaraj,C Fuller,R Bucala,I Jialal

doi:10.1016/s0022-2275(20)37563-5

Abstract

Much data support a role for both low density lipoprotein (LDL) oxidation and glycation in atherogenesis. While alpha-tocopherol decreases the oxidative susceptibility of LDL, its role in decreasing LDL glycation is unclear. Hence we tested the effect of alpha-tocopherol both in vitro and in vivo on LDL oxidation and glycation. LDL was isolated after enrichment of plasma with alpha-tocopherol. This resulted in a 2-fold increase in alpha-tocopherol in LDL (AT-LDL). During a 6-day incubation of control LDL (C-LDL) and AT-LDL with 25 mM glucose, there were no significant differences in the degree of glycation on days 1, 3, and 6. Also, apoB advanced glycosylation end product levels were not significantly different between C-LDL and AT-LDL. There was a progressive increase in the susceptibility of LDL to oxidation with increasing LDL glycation as evidenced by reduced lag time of copper-catalyzed LDL oxidation. However, AT-LDL was more resistant to copper-catalyzed oxidation. Similar findings were observed when the LDLs were incubated with endothelial cells. The data from the alpha-tocopherol supplementation study confirmed our in vitro findings that alpha-tocopherol significantly decreases oxidative susceptibility of LDL, but does not affect its glycation. Therefore, while glycation increases LDL oxidative susceptibility, alpha-tocopherol decreases the oxidation of glycated LDL but not LDL glycation.

Highlights

Much data support a role for both low density lipoprotein (LDL) oxidation and glycation in atherogenesis
Plausible modifications of lipoproteins, such as LDL, include non-enzymatic glycation and oxidation [1].Several lines of evidence support a role for glycated LDL and advanced glycosylation end products (AGE) in the genesis of the atherosclerotic lesion [2,3].numerous laboratories have documented that oxidatively modified LDL is pro-atherogenic and exists in vivo [4,5,6]. a-Tocopherol, a potent, lipid soluble antioxidant, has been shown to decrease the susceptibility of LDL to oxidation both in vitro and after in vivo supplementation [7, 8]
Two plausible mechanisms are LDL glycation and oxidation [1,2].Oxidation of LDL has been proposed as a key early step in the pathogenesis of atherosclerosis [4]

Summary

MATERIALS AND METHODS

Blood samples were collected for the study from healthy, non-smoking male or female subjects if they fulfilled the following criteria: not on any vitamin supplements for at least 6 months before entry; alcohol intake < 1 oz/day; normal fasting plasma glucose, hepatic and renal function tests; no evidence of malabsorption, pancreatic, or biliary diseases; not on estrogen, thyroxine, or non-steroidal anti-inflammatory drugs, and no acute medical condition for at least 3 months prior to participating in the study. In order to determine whether glycation interferes with the dye binding assay, LDL was incubated with 0 , 25, 50, and 100 mM glucose for 6 days at 37°C. The protein concentration of the LDL incubated in PBS and the LDL incubated with 100 mM glucose for 6 days differed by only 2.7% (n = 3 experiments). For measurement of apoB-AGE levels, C-LDL and AT-LDL at a concentration of 3 mg/ml were incubated with 25 mM glucose for 6 days. LDL oxidation, glycated plasma proteins, HbAlC, glycated LDL, and a-tocopherol levels in plasma and LDL were measured at baseline and after supplementation. The details of this in vivo supplemen-

96 Glycation

RESULTS

DISCUSSION

Findings

Weeks min