On the problem of dehydration and intracellular crystallization during freezing of cell suspensions.

The kinetic equation of the process of cell dehydration during freezing has been obtained. It is used to assess the degree of protoplasmic supercooling as a function of the cooling rate and cell parameters.The suggested model of dehydration cannot be applied to cells with permeability coefficients for water molecules more than 10^?5 cm/sec · bar, in particular to erythrocytes.The peculiarities of intracellular crystallization in red cells have been studied. The results show that red cells are likely to start freezing at cooling rates slower than those supposed from calculations of Mazur (9).     

Effects of Hydration on Steric and Electric Charge-Induced Interstitial Volume Exclusion—a Model

The presence of collagen and charged macromolecules like glycosaminoglycans (GAGs) in the interstitial space limits the space available for plasma proteins and other macromolecules. This phenomenon, known as interstitial exclusion, is of importance for interstitial fluid volume regulation. Physical/mathematical models are presented for calculating the exclusion of electrically charged and neutral macromolecules that equilibrate in the interstitium under various degrees of hydration. Here, a central hypothesis is that the swelling of highly electrically charged GAGs with increased hydration shields parts of the neutral collagen of the interstitial matrix from interacting with electrically charged macromolecules, such that exclusion of charged macromolecules exhibits change due to steric and charge effects. GAGs are also thought to allow relatively small neutral, but also charged macromolecules neutralized by a very high ionic strength, diffuse into the interior of GAGs, whereas larger macromolecules may not. Thus, in the model, relatively small electrically charged macromolecules, such as human serum albumin, and larger neutral macromolecules such as IgG, will have quite similar total volume exclusion properties in the interstitium. Our results are in agreement with ex vivo and in vivo experiments, and suggest that the charge of GAGs or macromolecular drugs may be targeted to increase the tissue uptake of macromolecular therapeutic agents.     

Natural terpenes: self-assembly and membrane partitioning

Monoterpenes (MTs) are highly hydrophobic substances present in essential oils. They cover a wide spectrum of biological effects with a membrane interaction as a common point. Here we studied the surface activity of camphor, cineole, thymol, menthol and geraniol, and their ability to reach and incorporate into model membranes affecting some features of their dynamic organization. All the MTs studied self-aggregated in water with critical micellar concentrations (CMC) between 3 and 8 microM. Their octanol-water and membrane-water partition coefficients were correlated with one another. They all penetrated in monomolecular layers of dipalmitoyl-phosphatildylcholine at the air-water interface, even at surface pressures (pi) above the equilibrium lateral pressure of bilayers; thymol exhibited the highest (61.3 mN/m) and camphor the lowest (37 mN/m) pi(cut-off) value. They affected the self-aggregation of Triton X-100, increasing its CMC from 0.16 mM in the absence of MTs up to 0.68 mM (e.g. for geraniol), and the topology of sPC vesicles, increasing its surface curvature, suggesting their location at the polar head group region of the membrane. The latter was supported by their ability to increase differentially the polarity of the membrane environment sensed by two electrochromic dyes. Dipole moment values (between 1.224 and 2.523 D) and solvation areas (between 80 and 97 A(2)) were calculated from their energy-minimized structures. The relative contribution of each experimental, theoretical and structural property to determine MTs' effects on membrane dynamics were evaluated by a principal component analysis.     

Changes of enzyme activity in lipid signaling pathways related to substrate reordering

The static fluid mosaic model of biological membranes has been progressively complemented by a dynamic membrane model that includes phospholipid reordering in domains that are proposed to extend from nanometers to microns. Kinetic models for lipolytic enzymes have only been developed for homogeneous lipid phases. In this work, we develop a generalization of the well-known surface dilution kinetic theory to cases where, in a same lipid phase, both domain and nondomain phases coexist. Our model also allows understanding the changes in enzymatic activity due to a decrease of free substrate concentration when domains are induced by peptides. This lipid reordering and domain dynamics can affect the activity of lipolytic enzymes, and can provide a simple explanation for how basic peptides, with a strong direct interaction with acidic phospholipids (such as beta-amyloid peptide), may cause a complex modulation of the activities of many important enzymes in lipid signaling pathways.