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1. Cellular Wounds

Regarding injury to cellular macromolecules, damaging physical forces are most likely to alter the folded structure of proteins resulting in loss of functionality. Examples include protein melting (thermal denaturation) in response to higher than physiologic temperature exposure (Tsong et al., 1994, 1999), electroconformational denaturation of ion channels and lipid bilayer electroporation following exposure to large transmembrane electrical potentials (Chen et al., 1994), and freeze induced protein damage (Rubinsky et al., 1992). Loss of protein architecture also can result from exposure to reactive chemical agents such as what commonly occurs as a result of excessive intracellular generation of reactive oxygen species (ROS). (Palmer et al., 1998) Reactive chemical injuries have in common the ability to alter the primary structure of macromolecules. In comparison to thermal burn and electrical injury, the major damaging effects on cells from ionizing radiation injury are initiated at the primary protein structure level. (Lee and Astumian, 1996) Altered macromolecules prevent their assembly into functional cellular structures thus leading to cellular wounds.

Damage to molecular assemblies, like the lipid bilayer, is perhaps the most common mode of cell wounding. (McNeil and Steinhardt, 1997) Loss of cell membrane integrity occurs at supraphysiologic temperatures (Bischof, 1995), in frostbite injuries (Rubinsky et al., 1992), in free-radical mediated radiation injury (Palmer et al., 1998), in barometric trauma (Fischer et al., 1997; McNeil and Steinhardt, 1997) and in electrical shock (Gaylor et al., 1988; Lee et al., 1992; Tsong and Su, 1999) and mechanical shear or crush forces (McNeil et al., 2000; Shi et al., 1999). Ischemia-reperfusion injury, which is mediated by the effects of ROS, is probably the most common cause and is a substantial factor in many common medical illnesses. (Palmer et al., 1998)

Self-organized membranes, like the lipid bilayer, are dynamically stabilized by their ability to self-assemble in an aqueous environment if the local concentration is above a critical concentration level. The most familiar example is the formation of bubbles when water containing a surfactant is agitated. In a pure surfactant membrane (i.e. a liposome), a sufficiently large enough wound to cause an ionically conducting pore will expand until the entire structure ruptures. (Taylor and Michael, 1984) Surface tension is responsible for causing the wound to expand. This is similar to a pinprick of a soap bubble. Because cell membranes also contain large proteins, some of which are anchored together in the intracellular and the extracellular space by other proteins, a wound in the membrane is prevented from complete rupture. Using freeze-fracture electron microscopy, Chang et al. (1990) demonstrated that stable structural defects (i.e., wounds) to occur in electro-porated cell membranes. Their studies suggest pore diameters might be in the range of 0.1 microns, which is large enough to pass most biomacromolecules.

Although the final result is a cell wound, the modes of cellular membrane injury are through different pathways. ROS produces wounding of the cell membrane through peroxidation of phospholipids and oxidative deamination of proteins. This alters lipid conformation and results in blebing followed by formations of membrane defects. Membrane electroporation results from the pull of water into the membrane by the supraphysiologic electric fields. Heating increases the kinetic energy of membrane amphiphilic lipids until their momentum overcomes the forces of hydration that act to hold the lipids within the membrane lamella. Under freeze conditions, ice nucleation in the cytoplasm can lead to factors which are very destructive to the cell membrane, including the mechanical disruption of the membrane by the ice crystal growth and the damaging effects of increasing salt concentration as the ice spreads and excludes ions. (Karrison, 1993) Abrupt barometric pressures can lead to acoustic wave disruption of the cell membrane (Fischer et al., 1997).