Reactivity of lecithin-cholesterol acyl transferase (LCAT) towards glycated high-density lipoproteins (HDL)

Abstract

Hyperglycaemia in diabetic patients results in non-enzymatic glycation of plasma proteins, including lipoproteins such as high-density lipoproteins (HDL). We studied the effects of in vitro HDL glycation on the activity of lecithin-cholesterol acyl transferase (LCAT), a key enzyme in HDL plasma metabolism. LCAT was prepared from non-diabetic subjects and HDL by sequential density ultracentrifugation (in the density range of 1.063–1.21 g/ml) from both diabetic and non-diabetic patients. HDL from non-diabetic patients were glycated in vitro by incubating lipoproteins with 100 mmol/l glucose for various times at 37°C with sodium cyanoborohydride as reducing agent. Glycation of HDL protein was quantified by measuring the percentage of derived amino acid residues using the TNBS assay. Kinetic parameters of LCAT were first determined using native HDL from non-diabetic patients and in vitro glycated HDL. With native HDL, K m and V max were 51.1 ± 4.2 μmol/l ( n = 8) and 12.9 ± 2.4 nmol/ml/h ( n = 8), respectively. Enzyme reactivity, calculated as the V max/ K m ratio, was 0.25 ± 0.04 h −1 ( n = 8). In the case of moderate glycation (derived residues < 30%; n = 19) a significant increase in both K m (18.2 ± 3.4%; mean ± S.D.) and V max (9.3 ± 2.4%) was observed. In contrast, with a high level of glycation (derived residues > 30%; n = 8), both parameters fell ( K m, 25 ± 6.3%; V max, 34.1 ± 3.3%). In addition, whatever the level of glycation, enzyme reactivity was lower in the presence of in vitro glycated HDL. This decrease in LCAT reactivity was not due to a peroxidative process nor to an alteration of the protein and lipid composition of in vitro glycated HDL. It could, however, be explained by glycation of lysine residues in apolipoprotein A-I, which is the most potent activator of LCAT. In a second series of experiments, native diabetic HDL preparations were used as LCAT substrate. No alteration in K m values was observed, but there was a significant decrease in both V max (28%) and enzyme reactivity (32%). This difference in K m and V max alterations between native diabetic HDL and in vitro glycated HDL with low levels of glycation might be explained by the impact of physiological modifications, other than glycation, which could differently affect the chemicophysical properties of HDL in diabetic patients. In conclusion, the decrease in LCAT reactivity observed with both native diabetic HDL and in vitro glycated HDL could affect the reverse cholesterol transport of HDL and thereby contribute to the atherosclerotic process in diabetic patients.