Distribution of C apolipoproteins between the vascular and lymph compartments of the rat

This study has investigated the kinetics of transfer of C apolipoproteins between the vascular and lymph compartments of the rat. Very-low-density lipoprotein, labeled with [125I]apolipoprotein C, was injected intravenously into lymph duct-cannulated rats and the redistribution of radioactivity between lymph and plasma followed at frequent intervals for 3 h. Equilibration between the two compartments was rapid (10-15 min), and thereafter removal from both compartments continued at similar rates. Specific radioactivity determinations showed that lymph C-III-0, C-III-3, and C-III-2,1 apolipoproteins rapidly reached values identical to those of corresponding plasma C apolipoproteins and the interrelationship between the curves were consistent with precursor-product relationships in which all, or most, of the product (lymph apolipoprotein C-III) was derived from the precursor (plasma). However, the specific radioactivity curves for C-II peptide did not cross; the lower value for lymph C-II apolipoprotein suggests that, unlike C-III apolipoproteins, a substantial proportion (approx. 40%) of lymph C-II peptide is not derived from the plasma compartment. The most likely source of the unlabeled lymph apolipoprotein C-II is synthesis and secretion from the intestine.     

Comparison of binding and degradation of high density lipoprotein by intestinal mucosal cells, fibroblasts and adrenal cortical cells in culture

In this study we have compared the binding and degradation of human high density lipoprotein (HDL3), devoid of apolipoprotein E, by rat intestinal (mucosal) and adrenal cells and by human fibroblasts in culture. Binding of HDL3 to adrenal and intestinal cells was characterised by saturable, specific processes whereas skin fibroblasts from normal humans did not convincingly demonstrate saturability and had a lower affinity and capacity compared with adrenal and intestinal cells. Post-receptor events also appeared to differ. Cells from the adrenal cortex and gut showed similar binding affinities for HDL3 but the capacity for binding and for degrading HDL3 was much higher with intestinal cells. The large amounts of HDL degraded by intestinal cells suggest a specific role for the gut in HDL catabolism, and that, in the rat, intestinal cholesterol may be derived from circulating HDL. Finally, it is suggested that rat adrenal cortical and intestinal mucosal cells possess surface receptors for HDL3 which differ from the LDL receptor.     

Rotational diffusion of human lipoproteins and their receptors as determined by time-resolved phosphorescence anisotropy

Time-resolved phosphorescence anisotropy has been used to assess the rotational dynamics of human serum lipoproteins labeled with phosphorescent probes of high triplet yield. Labeling the lipid phase of low density, very low density, and high density lipoproteins with an eosinyl fatty acid revealed the existence of two motions. The shorter time constant was attributed to motion of the chromophore within the lipoprotein particle, while the longer time constant represented the global tumbling of the particles in solution. The measured correlation times for this global motion were about twice those predicted from the Stokes-Einstein relationship. Covalent labeling of the apolipoproteins of the low and high density lipoproteins with erythrosin revealed the existence of segmental motion of labeled domains of the apolipoprotein within their respective particles. The correlation times for this motion were within the range 10-50 microseconds. The binding of low density lipoproteins to receptors on membranes isolated from the adrenal cortex resulted in a freezing of the global motion, but maintenance of the faster segmental motion of the labeled domains of the apolipoprotein. The experiments imply that in these membranes there is no global motion of the low density lipoprotein-receptor complex on the phosphorescence time scale. Similar results were found for the binding of high density lipoproteins to liver plasma membranes. The contributions of nonspecific binding of the labeled lipoproteins to the measured phosphorescence anisotropy were carefully assessed.     

Effect of cholesterol in suspension on the incorporation of phosphate into phospholipid by macrophages in vitro

Macrophages obtained from the peritoneal cavity of rabbits were incubated with phosphate-(32)P in order to investigate the synthesis of phospholipid by these cells. After 6 hr of incubation 0.25% of the phosphate added to the medium had been incorporated into phospholipid by the macrophages, mainly into lecithin and sphingomyelin, but partly also into phosphatidyl ethanolamine and inositol phosphatide. The addition of cholesterol to the macrophage suspensions increased the rate of incorporation by 20% with 1 mg of cholesterol added, and 44% with 2.5 mg added. The increase was similar for all the phospholipid fractions, and was not accompanied by any increase in oxygen uptake by the cells. The addition of carbon particles (as a specific check for phagocytic effects) had only a small effect on the rate of incorporation. The data provide support for the concept that cholesterol stimulates phospholipid synthesis by similar cells in the arterial wall during atherogenesis.     

Abnormal apolipoprotein composition in alcoholic hepatitis

Alcoholic hepatitis leads to major derangements in lipoprotein metabolism. This study defines the characteristics of the abnormal high density lipoprotein and very low density lipoprotein in relation to the severity of the disease. In severely affected subjects very low density lipoprotein apolipoproteins were deficient in apolipoprotein E and apolipoprotein C. The concentration of high density lipoprotein was markedly reduced, although the proportion of high density lipoprotein 1 was substantially elevated when compared to normal subjects. High density lipoproteins were deficient in apolipoprotein AI and apolipoprotein AII but enriched in apolipoprotein E, apolipoprotein E complexes and apolipoprotein C, and contained a mixture of particles. The high density lipoprotein of subjects with alcoholic hepatitis contained a high proportion of material which bound to heparin affinity columns. This bound fraction contained a group of particles rich in apolipoprotein E, apolipoprotein E complexes and apolipoprotein C and was deficient in apolipoprotein AI and apolipoprotein AII. Examination by electron microscopy showed the presence of both discoidal and spherical particles, which varied in concentration according to the severity of the disease. Another fraction of high density lipoprotein, not bound to heparin, contained reduced amounts of apolipoprotein AI and apolipoprotein AII, consisted of disc-shaped particles and showed a higher esterified: free cholesterol ratio than the other high density lipoprotein fraction.     

Metabolic heterogeneity in the formation of low density lipoprotein from very low density lipoprotein in the rat: evidence for the independent production of a low density lipoprotein subfraction.

The formation of low density lipoprotein (LDL) from very low density lipoprotein (VLDL) was studied after injecting 14C-radiomethylated or 125I-radioiodinated VLDL into rats. VLDL and LDL B apoprotein specific radioactivity time curves were obtained after tetramethylurea extraction of the lipoproteins. In all experiments, the specific activity of LDL B apoprotein did not intercept the VLDL curve at maximal heights, suggesting that not all LDL B apoprotein is derived from VLDL B apoprotein. Further subfractionation of LDL into the Sf 12-20, 5-12, and 0-5 ranges showed that most (65%) LDL B apoprotein was present in the Sf 0-5 fraction and that only a small proportion (6-15%) of this fraction was derived from VLDL. However, the curves obtained for the Sf 12-20 and 5-12 subfractions were consistent with a precursor-product relationship in which all of these fractions were derived entirely from VLDL catabolism. These results contrasted strikingly with similar data obtained for normal humans in which all LDL is derived from VLDL. In the rat, it appears that most of the B apoprotein in the Sf 0-5 range, which contains 65% of the total LDL B apoprotein, enters the plasma independently of VLDL secretion.     

Detecting recombination from gene trees

In this article, a method is proposed for detecting recombination in the sequences of a gene from a set of closely related organisms. The method, the Homoplasy Test, is appropriate when the sequences are rather similar, differing by 1%-5% of nucleotides. It is effective in detecting relatively frequent recombination between a set of rather similar strains, in contrast to previous methods which detect rare or unique transfers between more distant strains. It is based on the fact that, if there is no recombination and if no repeated mutations have occurred (homoplasy), then the number of polymorphic sites, v, is equal to the number of steps, t, in a most-parsimonious tree. If the number of "apparent homoplasies" in the most-parsimonious tree, h = t-v, is greater than zero, then either homoplasies have occurred by mutation or there has been recombination. An estimate of the distribution of h expected on the null hypothesis of no recombination depends on Se, the "effective site number," defined as follows: if ps is the probability that two independent substitutions in the gene occur at the same site, then Se = 1/ps. Se can be estimated if a suitable outgroup is available. The Homoplasy Test is applied to three bacterial genes and to simulated gene trees with varying amounts of recombination. Methods of estimating the rate, as opposed to the occurrence, of recombination are discussed.      

Purification and characterization of two high-density-lipoprotein-binding proteins from rat and human liver.

Newly absorbed retinol is transported in association with chylomicrons and their remnants. In addition, after intake of high doses of retinol, significant amounts are also found in low-density lipoprotein (LDL). As both chylomicron remnants and LDL may be taken up by cells via the LDL receptor, and retinoids inhibit proliferation of some leukaemic cells, we have studied the uptake of retinol in leukaemic cells via the LDL-receptor pathway. HL-60 cells contain saturable binding sites for LDL. The binding of LDL to its receptor has a dissociation constant of about 3.2 x 10(-9) M, and the number of receptors per cell was calculated to be about 2700. Uptake of 125I-LDL by HL-60 cells was increased 2-fold by preincubating the cells with mevinolin. The presence of specific receptors for LDL on HL-60 cells was further confirmed by the finding that exogenous LDL cholesterol was able to up-regulate the ACAT (acyl-CoA: cholesterol acyltransferase) activity of HL-60 cells. We then tested the uptake of retinyl ester in leukaemic cells via the LDL-receptor pathway. HL-60 cells were incubated with LDL or chylomicron remnants labelled with [3H]retinyl palmitate. Uptake of retinyl ester associated with both LDL and chylomicron remnants was observed. Furthermore, the presence of excess LDL decreased the uptake by 75-100%, supporting the hypothesis that the uptake of retinyl ester occurred via the LDL receptor in HL-60 cells.