Correlation between serum lecithin-cholesterol-acyltransferase activity and erythrocyte lipid content during experimental liver damage

Erythrocyte membrane alterations and plasma lipids in patients with chylomicronemia and in Tangier disease

The relationship between erythrocyte membrane structural and functional alterations and plasma lipids was studied in three patients with chylomicronemia due to either lipoprotein lipase (LPL) deficiency, apo C-II deficiency (in an individual who also suffers from thalassemia minor) or coexistent diabetes mellitus (and decreased LPL activity) and in a patient with Tangier disease. All of the patients' erythrocytes had significantly elevated phosphatidyl-choline (PC): sphingomyelin (Sph) ratios (most marked in the patient with Tangier disease). Major differences were observed in the PC: Sph ratios of erythrocytes and plasma. The pattern of changes in erythrocyte membrane enzyme activities differed despite similarities in the lipid composition of the erythrocytes. The changes in osmotic fragility (OF) were inversely related to the membrane cholesterol:phospholipid ratio. An even stronger negative correlation was found between OF at the lowest NaCl concentrations and the activities of both Na+,K+- and Mg++-ATPases. The ratio of total: surface sulfhydryl titres also correlated significantly with OF.

Lecithin: cholesterol acyl transferase (LCAT)

Esterification of cholesterol in plasma is mediated by LCAT. The mechanism of the three reactions catalysed by the enzyme is beginning to be understood. LCAT has been purified from human plasma and partially characterized. The enzyme is closely associated with HDL and exists most likely as a complex with its activator apo A-I and apo D. Antibodies were raised against LCAT and the enzyme concentration in plasma has been estimated to range between 4.5 and 8.0 mg/L. In patients with familial LCAT deficiency only trace amounts or no LCAT protein is found. Heterozygotes for this disorder have approximately half the normal amount of the enzyme. LCAT reactivity is essential for normal lipoprotein metabolism and for a proper equilibrium between tissue and plasma cholesterol.