Enzyme replacement via liposomes. Variations in lipid compositions determine liposomal integrity in biological fluids.

Liposomes survive exposure to biological fluids poorly, extruding trapped enzymes, drugs, or solutes upon interaction with serum or plasma constituents. We have quantified the disruptive effects of human serum on liposomes and have studied whether various modifications in their phospholipid composition might produce liposomes with an increased carrier potential for application in vivo. Multilamellar liposomes (phosphatidycholine 70:dicetyl phosphate 20:cholesterol 10) were prepared with 3H-labeled phosphatidylcholine as the lipid phase marker and [14C]inulin and horseradish peroxidase as aqueous phase markers. Gel exclusion chromatography showed that 32 +/- 3% of [14C]inulin and 27 +/- 7% of horseradish peroxidase were lost after 1 h incubation with 10% (v/v) human serum. Loss of aqueous solutes was reduced to 20 +/- 5%/h and 17 +/- 2%/h, respectively, after treatment with decomplemented serum (56 degrees C, 30 min). Loss induced by serum was concentration and time dependent: to 57 +/- 2% at 1 h and 67 +/- 14% at 24 h, with 50% serum; plasma was slightly less perturbing whereas human serum albumin was not at all disruptive. By incorporating sphingomyelin (35 mol%) into multilamellar liposomes, the leakage of [14c]-inulin in the presence of 10% serum was reduced to 12 +/- 4%/h; increasing the molar percentage of cholesterol to 35% also stabilized the lipid bilayers, reducing leakage to 20 +/- 7%/h. Both small and large unilamellar vesicles could not be stablilized against serum-mediated leakage by the incorporation of sphingomyelin. The data suggest that cholesterol and sphingomyelin enhance liposomal integrity in the presence of serum or plasma and promise to yield enhanced survival of drug-laden lipid vesicles in biological fluids in vivo.     

General architecture of the alpha-helical globule

A model is presented for the arrangement of alpha-helices in globular proteins. In the model, helices are placed on certain ribs of "quasi-spherical" polyhedra. The polyhedra are chosen so as to allow the close packing of helices around a hydrophobic core and to stress the collective interactions of the individual helices. The model predicts a small set of stable architectures for alpha-helices in globular proteins and describes the geometries of the helix packings. Some of the predicted helix arrangements have already been observed in known protein structures; others are new. An analysis of the three-dimensional structures of all proteins for which co-ordinates are available shows that the model closely approximates the arrangements and packing of helices actually observed. The average deviations of the real helix axes from those in the model polyhedra is +/- 20 degrees in orientation and +/- 2 A in position (1 A = 0.1 nm). We also show that for proteins that are not homologous, but whose helix arrangements are described by the same polyhedron, the root-mean-square difference in the position of the C alpha atoms in the helices is 1.6 to 3.0 A.     

Type II pneumocyte changes during hyperoxic lung injury and recovery

Adult rabbits exposed to 100% O2 for 64 h and then returned to room air for up to 200 h, develop a lung injury characterized by decreased levels of alveolar surfactant followed by a rebound recovery. In the present study we isolated alveolar type II cells from rabbits at various times during hyperoxic exposure and recovery and measured rates of phosphatidylcholine (PC) synthesis, cellular lipid content, and the specific activity of glycerol 3-phosphate (G-3-P) acyltransferase, an enzyme that catalyzes one of the early reactions in phosphoglyceride biosynthesis. These biochemical parameters were compared with measurements of cell size and cell cycle phase by laser flow cytometry. Results showed that alterations in alveolar phospholipid levels in vivo correlated consistently with cellular lipid metabolic changes measured in isolated type II pneumocytes. In particular, alveolar pneumocytes isolated from lungs of rabbits exposed to 100% O2 for 64 h exhibited a 60% decrease in PC synthesis, cell lipid content, and G-3-P acyltransferase activity. All variables then followed a pattern of recovery to normal and ultimately supranormal levels beginning at approximately 3 days postexposure, at which point there was also a measured increase in the number of type II cells in S phase. These findings suggest that O2-induced changes in type II cell surfactant biosynthesis may account, at least in part, for observed changes in lung phospholipid levels in vivo.     

Alteration of somatostatin but not growth hormone-releasing factor pituitary binding sites in obese Zucker rats.

The present study was designed to determine whether the diminution of growth hormone (GH) secretion that occurs in obese Zucker rats is related to alterations of GH-releasing factor (GRF) or somatostatin (SRIF) pituitary binding sites. Cold saturation studies were performed in pituitary homogenates of 4-month-old lean and obese rats, using [125I-Tyr10]hGRF(1-44)NH2 as radioligand and [127I-Tyr10]hGRF-(1-44)NH2 as competitor, and in pituitary membrane preparations, using [125I-Tyr0, D-Trp8]SRIF14 as radioligand and [127I-Tyr0, D-Trp8]SRIF14 as competitor. In lean rats, analysis of the curves by the Ligand program revealed the presence of two distinct classes of GRF binding sites, the first being of high affinity (0.74 +/- 0.11 nM) and low capacity (118 +/- 31 fmol/mg protein), the second being of lower affinity (880 +/- 240 nM) and higher capacity (140 +/- 35 pmol/mg protein), and of a single class of SRIF binding sites (affinity: 0.40 +/- 0.12 nM; capacity: 24 +/- 6 fmol/mg protein). In obese rats, no difference was observed in GRF binding parameters for both classes of sites, but the concentration of somatostatin binding sites was reduced by 67% when compared to their lean littermates. These findings suggest that the SRIF pituitary receptors are down-regulated in obese Zucker rats and indicate that no alteration of GRF pituitary binding sites contribute to the blunted GH secretion observed in this model of obesity.     

Channels formed by colicin E1 in planar lipid bilayers are large and exhibit pH-dependent ion selectivity

Summary The E1 subgroup (E1, A, Ib, etc.) of antibacterial toxins called colicins are known to form voltage-dependent channels in planar lipid bilayers. The genes for colicins E1, A and Ib have been cloned and sequenced, making these channels interesting models for the widespread phenomenon of voltage dependence in cellular channels. In this paper we investigate ion selectivity and channel size—properties relevant to model building. Our major finding is that the colicin E1 channel is large, having a diameter ofat least 8 Å at its narrowest point. We established this from measurements of reversal potentials for gradients formed by salts of large cations or large anions. In so doing, we exploited the fact that the colicin channel is permeable to both cations and anions, and its relative selectivity to them is a functions and anions, and its relative selectivity to them is a function of pH. The channel is anion selective (Cl^– over K⁺) in neutral membranes, and the degree of selectivity is highly dependent on pH. In negatively charged membranes, it becomes cation selective at pH's higher than about 5. Experiments with pH gradients cross the membrane suggest that titratable groups both within the channel lumen and near the channel ends affect the selectivity. Individual E1 channels have more than one open conductance state, all displaying comparable ion selectivity. Colicins A and Ib also exhibit pH-dependent ion selectivity, and appear to have even larger lumens than E1.