REACTIVITY OF LECITHIN-CHOLESTEROL ACYL TRANSFERASE (LCAT) TOWARDS GLYCATED HIGH-DENSITY-LIPOPROTEINS (HDL)

Hyperglycaemia in diabetic patients results in non-enzymatic glycation of plasma proteins, including lipoproteins such as high-density lipop roteins (HDL). We studied the effects of in vitro HDL glycation on the activity of lecithin-cholesterol acyl transferase (LCAT), a key enzym e in HDL plasma metabolism. LCAT was prepared from non-diabetic subjec ts and HDL by sequential density ultracentrifugation (in the density r ange 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 l ipoproteins with 100 mmol/l glucose for various times at 37 degrees C with sodium cyanoborohydride as reducing agent. Glycation of HDL prote in was quantified by measuring the percentage of derived amino acid re sidues using the TNBS assay. Kinetic parameters of LCAT were first det ermined using native HDL from non-diabetic patients and in vitro glyca ted HDL. With native HDL, K-m and V-max were 51.1 +/- 4.2 mu mol/l (n = 8) and 12.9 +/- 2.4 nmol/ml/h (n = 8), respectively. Enzyme reactivi ty, 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 g lycation (derived residues > 30%; n = 8), both parameters fell (K-m, 2 5 +/- 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 gly cated HDL. This decrease in LCAT reactivity was not due to a peroxidat ive 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 ac tivator of LCAT. In a second series of experiments, native diabetic HD L preparations were used as LCAT substrate. No alteration in K-m value s 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 alter ations 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 th e chemicophysical properties of HDL in diabetic patients. In conclusio n, the decrease in LCAT reactivity observed with both native diabetic HDL and in vitro glycated HDL could affect the reverse cholesterol tra nsport of HDL and thereby contribute to the atherosclerotic process in diabetic patients.