Diabetes and lipoprotein receptors

The relationship of chemical modification of membrane proteins and plasma lipoproteins to reduced membrane fluidity of erythrocytes from diabetic subjects

The significance of the two most common hallmarks of the diabetic state, hyperglycaemia and hyperlipidaemia, was investigated in terms of disorders of cell membrane dynamics. In order to examine whether the alterations in cell membrane lipid bilayer dynamics are somehow related to protein chemical modifications in plasma low-(LDL) and high-density lipoproteins (HDL) and blood cell membranes, we compared 19 poorly controlled diabetic subjects with 19 age- and sex-matched controls. The extent of (non-enzymatic) glycation, lipid peroxidation and the cholesterol/phospholipid ratio were increased in plasma low density lipoproteins and high density lipoproteins from diabetic patients. The mean steady-state fluorescence polarization values in 1,6-diphenyl-1,3,5-hexatriene-labelled isolated erythrocyte membranes from diabetic subjects were significantly greater than from control subjects (0.186 +/- 0.008 vs 0.173 +/- 0.006, p < 0.001); the fluorescence polarization values in erythrocyte membranes from diabetic and control subjects positively correlated with the extent of membrane protein glycation, lipid peroxidation and the cholesterol content. The cholesterol to phospholipid molar ratios in low density lipoproteins and high-density lipoproteins from diabetic and control subjects correlated significantly with the fluorescence polarization values in erythrocyte membranes from these subjects. Furthermore, the extent of glycation of low density lipoproteins appears to be strongly correlated with the extent of lipoprotein lipid peroxidation (r = 0.789, p < 0.001). The atherosclerotic potential of plasma lipoproteins in diabetes mellitus was discussed in terms of membrane and plasma protein chemical modifications.

Glycated low density lipoprotein catabolism is increased in rabbits with alloxan-induced diabetes mellitus

Hyperglycaemia in diabetes mellitus is responsible for the process of non-enzymatic glycosylation of different proteins. Since we did not find elevated glycated apolipoprotein B levels in diabetic patients, an altered glycated apolipoprotein B metabolism was suspected in diabetic patients. Experiments in normal rabbits showed that non-reductive (in vitro) glycated low density lipoprotein (gly-LDL) was cleared at a slower rate than control LDL and thus stayed longer in the circulation (vascular mean residence time: 10 vs 8 h, p less than 0.001). The body mean residence time for gly-LDL was 22 h vs 17 h for control LDL. In diabetic animals the catabolic parameters of both LDL preparations changed towards a faster clearance, the effect being greatest for gly-LDL (total mean residence times of gly-LDL pre-diabetic: 19 h, diabetic: 16 h; control LDL pre-diabetic and diabetic: 14 h). The difference in clearance between glycated and control LDL was thus strongly reduced. Virtually no antibody complexed to gly-LDL could be measured. The results suggest an increased activity of the non-receptor mediated pathway in diabetes mellitus, possibly co-responsible for an increased atherosclerotic risk.

A simple method for the measurement of total and glycated apolipoprotein B and its relevance to apolipoprotein-B metabolism in diabetes mellitus

By the combination of boronate affinity chromatography on micro columns and immunoturbidimetry, a method was developed for the measurement of glycated apoliprotein B (Apo-B). The overall precision of the method was 11%. The sensitivity was 0.3 mg/l Apo-B or 0.05% glycated Apo-B, which is far below physiological levels. The recovery after column separation was 95 +/- 7%. In vitro incubation of plasma with 90 mmol/l D-glucose showed an increase in the percentage glycated Apo-B of 0.58% per hour. The simplicity of the method makes it suitable for screening purposes. Non-diabetic patients (n = 21) and diabetic patients (n = 35) were screened for glycated and total Apo-B. Total Apo-B in diabetics (1.02 +/- 0.53 g/l) was increased (controls: 0.75 +/- 0.18 g/l; p less than 0.05) and a positive correlation between HbA1 and Apo-B (r = 0.36, p less than 0.05) was present in this group. However, no significant difference of glycated Apo-B was measured between diabetics and controls (10.4 +/- 2.8 and 11.5 +/- 1.4%, respectively). The possible implications of these findings are discussed.

Treatment of lipid disorders in diabetes mellitus

One reason for premature atherosclerosis in patients with diabetes mellitus is abnormal lipid metabolism. This article discusses the plasma lipid disorders associated with diabetes mellitus, and how to apply the new guidelines from the National Cholesterol Education Program for treatment of hypercholesterolemia in patients with diabetes mellitus.