Cholesterol metabolic heterogeneity

F5. Fainaru, M., Mahley, R. W., Hamilton, R. L., and Innerarity, T. L., Structural and metabolic heterogeneity of (J-very low density lipoproteins from cholesterol-fed dogs and from humans with Type III hyperlipoproteinemia. J. Lipid Res. 23, 702-714 (1982). 

Heterogeneous reaction (Section 6 1) A reaction involving two or more substances present in different phases Hydro genation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst Heterolytic cleavage (Section 4 16) Dissociation of a two electron covalent bond in such a way that both electrons are retained by one of the initially bonded atoms Hexose (Section 25 4) A carbohydrate with six carbon atoms High density lipoprotein (HDL) (Section 26 11) A protein that carries cholesterol from the tissues to the liver where it is metabolized HDL is often called good cholesterol Histones (Section 28 9) Proteins that are associated with DNA in nucleosomes... 

Myelin components exhibit great heterogeneity of metabolic turnover. One of the novel characteristics of myelin demonstrated in early biochemical studies was that its overall rate of metabolic turnover is substantially slower than that of other neural membranes [1]. A standard type of experiment was to evaluate lipid or protein turnover by injecting rat brains with a radioactive metabolic precursor and then follow loss of radioactivity from individual components as a function of time. Structural lipid components of myelin, notably cholesterol, cerebro-side and sulfatide, as well as proteins of compact myelin, are relatively stable, with half-lives of the order of many months. One complication in interpreting these studies is that the metabolic turnover of individual myelin components is multiphasic - consisting of an initial rapid loss of radioactivity followed by a much longer slower loss. 

The studies that led to the lipid hypothesis measured plasma total cholesterol concentration. Cholesterol is insoluble in aqueous solution and needs to be combined with protein for transport in blood. These plasma lipoproteins are large heterogeneous aggregates that have different physical properties, such as density, chemical composition and metabolic function (Gurr et al., 2002). 

HDL particles are quite heterogeneous, encompassing a range of sizes and densities they also contain multiple surface apolipoproteins (Apo Al, All, ATV, AV, CII, cm, D, E, F, H, J, L, M) varying concentrations of cholesterol and phospholipids, different types of lipid transfer proteins (CETP, PLTP) as well as various antioxidant or pro-oxidant enzymes (PONl, LCAT, GPx, PAF-AH), now recognized as significant factors in the balance of pro- and antiatherogenic metabolism [137,138]. Several properties of functional and dysfunctional HDL have been compared in Table 17.1. 

---