Colin Green

The transfer of lipids between human α-lipoprotein and erythrocytes

Abstract 1. 1. The transfer of palmitic acid and sterol between human α-lipoprotein and erythrocytes has been studied. 2. 2. Human erythrocytes take up palmitic acid from α-lipoprotein until all binding sites are filled. The process is very much more rapid if the lipoprotein also extra cholesterol. 3. 3. 38% of the endotenous unesterified sterol of human α-lipoprotein does not exchange with that of rat erythrocytes. 4. 4. [ 14 C]Cholesterol taken up by the lipoprotein after dispersion on Celite exchanges with that of human erythrocytes but it does not behave in exactly the same way as the endogenous sterol. 5. 5. When α-lipoprotein is labelled with both cholesterol and palmitic acid and then incubated with human erythrocytes, each interferes with the transfer, to the cells, of the other.

The binding of polyamines to phospholipid bilayers

Previous studies have shown that amine groups are ototoxic. The interaction between different polyamines and phospholipid vesicles was studied using vesicle aggregation and fluorescence techniques (DPH and ANS as the fluorescence probes). The results showed that the interaction between polyamines (spermine, spermidine and 1,3-diaminopropane) and acidic phospholipids (PS, PE, PI and PIP2) is an ionic one. The polyamine with the highest positive charges and the phospholipid with the highest content of negative groups showed the strongest ionic interaction. There was no indication of any hydrophobic interaction within the phospholipid bilayer. The strong interaction between amine groups and PIP2 support the proposal that the latter is crucially involved in aminoglycoside toxicity in the inner ear and kidney.

The uptake of chylomicron fatty acids by isolated liver cells

Abstract 1. 1. Rats were fed [14C]tripalmitin and guinea-pigs injected with 14C-labelled chylomicrons and the livers examined at various time intervals. Most of the 14C was associated with the individual liver cells but up to 43% of the total in the tissue could be free of the cells. 2. 2. Isolated rat-liver cells can bind chylomicrons to the extent of 40 μg lipid per mg tissue N. 3. 3. The bound chylomicrons are hydrolysed to give free fatty acids. 4. 4. The fatty acids produced are firmly bound but can be removed by fatty acid-free albumin, suggesting that hydrolysis occurs at the cell surface. 5. 5. The hydrolysis of chylomicron triglyceride is not inhibited by protamine sulphate or by heating the cells at 60°; nor is it stimulated by taurocholate.

The dispersion of cholesterol with phospholipids and glycolipids

Of the polar lipids studied (phospholipids and glycolipids), only phosphatidylcholine and sphingomyelin can disperse in water with up to 2 mol cholesterol/mol polar lipid. However, mixtures of phosphatidylethanolamine with small amounts of phosphatidylcholine and mixed lipids from mitochondria and myelin will also form sterol-rich dispersions. Steroids in which the 3beta-OH group is replaced by an oxo function do not form such steroid-rich dispersions. Electron microscopy and optical rotatory dispersion (ORD) show that sterols disperse with cerebrosides and gangliosides to form cylindrical structures with the regions around C atoms 3 and 7 of the sterol in less polar environments than those they occupy in phospholipid liposomes. It is proposed that choline-containing phospholipids facilitate entry of sterol molecules into the outer leaflet of cell surface membranes but that the phospholipid composition itself will not give rise to an asymmetric distribution of sterol in membranes with a high cholesterol content.

Properties of a lipase of rat-liver parenchymal cells

Abstract 1. 1. The properties of the lipase of rat-liver parenchymal cell plasma membrane have been investigated. 2. 2. When chylomicron triglycoride is hydrolysed, very little di- or monoglyceride is produced. 3. 3. Protamine sulphate causes increased binding of chylomicrons but does not affect the final level of triglyceride hydrolysis. This level is also unaffected by NaCl (1 M) and DFP (10 −4 M) but is increased by taurocholate (1 mg/ml). It is also increased if the cells are first heated to 60° for 10 min. 4. 4. The lipase is extracted by saline solutions in an inactive form which is reactived by heparin. Heparin may also stimulate the activity of the native enzyme. 5. 5. Both the native enzyme and the saline-extracted, heparin-activated enzyme have pH optima of 4.0–4.5. The latter has a ‘ K m ’ of 2.4 · 10 −4 .

The binding of hormones and related compounds by normal and cholesterol-depleted plasma membranes of rat liver☆

Abstract The effects of removing part of the unesterified cholesterol from rat liver plasma membranes upon their ability to bind a number of different hormones and related compounds has been studied. Amongst steroids, the more polar compounds containing an 11 βOH group (dexa-methasone, cortisol, corticosterone) are bound to a greater extent by cholesterol-depleted membranes than by control membranes whilst the less polar compounds (deoxy-corticosterone, progesterone, testosterone, diethylstilboestrol) are bound to a lesser extent. The increase in binding of cortisol is related to the degree of cholesterol depletion. In comparison experiments using rat liver mitochondrial preparations, which normally possess a very low level of cholesterol, it is found that when this level is raised, the binding of corticosterone decreases and that of testosterone increases. Binding of adrenaline and thyroxine by cholesterol-depleted plasma membranes is greater than that of controls whereas the binding of peptide hormones (insulin, glucagon, oxytocin, vasopressin) is much lower. The results suggest that membrane sterols play an important part in regulating the uptake of biologically active compounds.

Erythrocyte membrane cholesterol levels and their effects on membrane proteins

The susceptibility of the band 3 protein of the erythrocyte membrane to proteolytic digestion at either surface of the membrane was not altered when the membrane cholesterol level was increased by 65-103%. Cross-linking of the major membrane proteins by o-phenanthroline . Cu, glutaraldehyde, dimethylsuberimidate and dimethyladipimidate was also unaffected.

Transfer of cholesterol between different regions of the rat hepatocyte surface membrane

Abstract 1. 1. Cholesterol, newly-synthesized from [14C]-mevalonic acid, took at least 35 min to become evenly distributed around the rat hepatocyte plasma membrane. 2. 2. [14C]cholesterol from injected liposomes entered the sinusoidal region of the membrane more rapidly than it was transferred to other regions, but transfer to other regions did occur. 3. 3. Liposomes containing [14C]cholesterol and [3H]phosphatidylcholine were appreciably adsorbed to the sinusoidal regions, but lipids which entered the bilayer by exchange or fusion migrated readily to other regions of the plasma membrane. 4. 4. Tight junctions do not appear to act as a complete barrier to diffusion of sterol through the bilayer.

Cholesterol transfer from rat, human and sheep erythrocytes

1. The spontaneous transfer of cholesterol out of sheep, rat and human erythrocyte membranes was measured. 2. The rates of cholesterol transfer did not correlate with the very different levels of sphingomyelin in the membranes. 3. Cholesterol transferred at similar rates out of vesicles made of lipids extracted from the three types of erythrocyte. 4. The results are discussed in relation to the proposal that cholesterol and sphingomyelin are closely associated in cell surface membranes.