P.R. Turner

1H NMR studies of human blood plasma assignment of resonances for lipoproteins

Single‐pulse and Hahn spin‐echo 500 MHz 1H NMR spectra of human blood plasma and isolated chylomicrons, VLDL, LDL and HDL are reported. The comparison has enabled specific assignments to be made for the resonances of individual lipoproteins in the CH2 and CH3 (fatty acid), and NMe+ 3 (phospholipid choline head group) regions of the spectra of plasma (0.8–1.3 and ∼ 3.25 ppm, respectively). Fasting, and freeze‐thawing of plasma samples led to marked changes in the intensities and linewidths of lipid resonances. Analysis of lipid resonances in the spectra of plasma in terms of individual lipoproteins may shed new light on many conditions of clinical and biochemical interest.

Modified plasma-derived lipoproteins in human atherosclerotic plaques

Low density lipoproteins extracted from surgical specimens of human atherosclerotic plaques (A-LDL) showed altered electrophoretic mobility indicating a greater negative charge than that of plasma LDL (P-LDL). A-LDL but not P-LDL showed high affinity binding/degradation by human monocyte-derived macrophages; this was inhibited by acetylated LDL but not by native P-LDL. Following injection of 125I-labelled autologous P-LDL prior to reconstructive arterial surgery, polyacrylamide and agarose gel electrophoresis of A-LDL extracted from arterial intima showed that the A-LDL and its apolipoprotein B moiety were derived from P-LDL; the electrophoretic mobility of the product A-LDL was greater than that of native P-LDL. The compositions of arterial intermediate density lipoprotein (A-IDL) and A-LDL differed from those obtained from human plasma intermediate density lipoprotein (P-IDL) and P-LDL. A-IDL showed a reduced triglyceride content and increased esterified and unesterified cholesterol. Although the total cholesterol content of A-LDL was similar to that of P-LDL, there was an increase in unesterified cholesterol and a decrease of cholesteryl ester. These studies indicate that LDL extracted from human atherosclerotic plaque is derived from and modified from P-LDL in vivo. Compared with native P-LDL, A-LDL showed differences in charge and composition, associated with its high affinity binding by the acetyl LDL receptor of human macrophages.

Metabolic studies on the hypolipidaemic effect of guar gum

The hypolipidaemic effect of guar gum (30 g/day) was examined in a double blind placebo-controlled crossover study in 9 patients with primary hyperlipidaemia. The treatment periods were of six weeks duration. Cholesterol levels in low density lipoprotein (LDL) were decreased by 11.5% and in intermediate density lipoprotein (IDL) by 10.7%. Plasma cholesterol levels were reduced by 9.6% (P less than 0.05). Kinetic studies using autologous 125I-labelled LDL showed a decrease of 21.6% in plasma LDL apo B pool size (P less than 0.05) that resulted from a 39.1% increase in its fractional rate of catabolism. The kinetic effects of guar gum on LDL metabolism appear similar to that of bile acid binding resins in that LDL apo B fractional catabolism is greatly increased while there is a slight increase in production rate.

Hereditary hyperlipidemia and atherosclerosis in the rabbit due to overproduction of lipoproteins. II. Preliminary report of arterial pathology.

A genetically determined hyperlipidemic strain of New Zealand White rabbit that has features in common with combined familial hyperlipidemia in humans has been identified. The morphologic findings in a few animals fed a normal chow diet are reported. These consisted of macroscopically visible aortic intimal elevations found in the greatest number in the descending thoracic aorta. The plaques showed the presence of a cell population consisting of modified smooth muscle cells and lipid-laden macrophages. The lesion bases were necrotic and acellular, and some showed the presence of dystrophic calcification. Scanning electron microscopy revealed numerous monocytes attached to the endothelium. Endothelial defects were common, and these were filled with swollen and “ruffled” macrophages. Transmission electron microscopy confirmed the presence of lipid-laden cells penetrating between adjacent endothelial cells. These findings resemble those reported in a number of different animal species after dietary induction of hyperlipidemia. This strain is a useful new model for the study of atherogenesis.

Hereditary hyperlipidemia in the rabbit due to overproduction of lipoproteins. I. Biochemical studies.

An inherited metabolic disorder in a strain of New Zealand White rabbits, characterized by marked hypercholesterolemia (394 ±100 mg/dl), with moderately elevated or normal triglyceride levels is described. Low density lipoprotein (LDL), intermediate density lipoprotein (IDL) and very low density lipoprotein (VLDL) cholesterol levels were increased. VLDL and IDL, and to a lesser extent LDL, had increased free cholesterol and esterified cholesterol content, and triglyceride content was reduced. Kinetic studies with 131I and 125I-labelled rabbit lipoproteins showed a marked increase in production rates of VLDL apo B and LDL apo B. LDL cholesterol levels were directly related to LDL apo B production rate (r = 0.938, p < 0.001). Both in hypercholesterolemic and normal rabbits injected with labelled VLDL, the specific activity-time curves of VLDL apo B and LDL apo B did not intersect, indicating that LDL apo B was in part derived from sources other than VLDL. No defect was demonstrated in receptormediated catabolism of LDL by cultured skin fibroblasts from hyperlipidemic animals. The fractional catabolic rate of LDL apo B was subnormal, but increased when the expanded LDL apo B pool size was reduced by exchange transfusion; the low fractional catabolism may therefore be attributable, at least in part, to saturation of LDL receptors consequent upon the increased pool size of LDL. The hyperlipidemia in this strain of rabbits may be unique in that the underlying mechanism appears to be overproduction of VLDL and LDL.

Hypocholesterolaemia and non-cardiovascular disease: metabolic studies on subjects with low plasma cholesterol concentrations.

Some epidemiological studies have suggested an inverse relation between serum cholesterol concentration and mortality from cancer. Two hypotheses that might explain such a relation were investigated. To assay potentially deleterious effects of hypocholesterolaemia on cell membranes the lipid content and fluidity of blood mononuclear cells were measured in healthy male volunteers with a wide range of serum cholesterol concentration (3.2-10.0 mmol/l (124-387 mg/100 ml)). Fluidity, unesterified cholesterol content, and the ratio of cholesterol to phospholipid were unrelated to serum cholesterol and to low density lipoprotein cholesterol concentrations. Similar measurements were made on fibroblasts and mononuclear cells incubated with a range of concentrations of low density lipoprotein; fluidity was altered only at extremely low concentrations, suggesting that changes in cell membranes are unlikely to occur at serum cholesterol concentrations attainable by dietary or drug treatment of hyperlipidaemia. In the same population direct relations were confirmed between low density lipoprotein concentration and plasma concentrations of retinol and beta carotene. This is compatible with the suggestion that an association between low cholesterol concentration and cancer may be secondary to a relation between low retinoid concentrations and cancer.

METABOLIC STUDY OF VARIATION IN PLASMA CHOLESTEROL LEVEL IN NORMAL MEN

Variation in plasma cholesterol in normal populations underlies a 2-5 fold range of risk of coronary heart disease. The metabolic basis of this variation was studied in a healthy population recruited from a general-practice list. Compared with men in the modal decile of plasma cholesterol, those in the top decile maintained their high levels by overproduction of low-density lipoprotein (LDL). Men in the bottom decile of plasma cholesterol had low levels of LDL, resulting chiefly from a greater rate of fractional catabolism of this lipoprotein; and blood mononuclear cells from men in the bottom decile catabolised LDL more rapidly in vitro than did cells from other men. Intake of dietary lipids by men in the top, modal, and bottom deciles did not differ significantly. Different mechanisms appear to underlie high and low levels of plasma cholesterol in a normal population.

Associations Between Plasma Cholesterol, Cell Membrane Composition and Fluidity, and Retinoid Concentrations in a Normal Male Population

Reduction of plasma cholesterol levels by diet and, when appropriate, by other means is a widely accepted component of programmes to reduce coronary heart disease risk1. In preventive medicine risk/benefit considerations are even more exacting than in therapy2; hence any suggestion that low cholesterol levels might have adverse effects deserves close scrutiny. Some prospective studies have shown an increased risk of total cancer mortality3 or colon cancer deaths4 in the lowest quintile of plasma cholesterol, though this was not observed in other studies5,6.