Ivan Ivanov
2014 Science & Technologies   unpublished
High temperature is the main physical factor causing damage of tissues, or burns, due to thermal injury and electric shock [1]. At cellular level these burns cause necrosis and, in rare cases, apoptosis. On the other hand, hyperthermia is used as a method for healing tumors. However, the primary targets whose destruction by heat leads to cellular death are not well understood. Temperatures between 45 and 58 o C cause thermal hemolysis of erythrocytes, i.e., out leakage of hemoglobin. Thermal
more » ... oglobin. Thermal hemolysis exhibits all the features peculiar to thermal necrosis of cells. Another type of hemolysis occurs at other non-physiological conditions, the so called eryptosis [2], which demonstrates the main requisites of apoptosis. In this regard, thermal hemolysis of human and mammal erythrocytes provides useful cellular model with biological and medical importance. The aim of this review is to present the main problems in our understanding of thermal hemolysis of enucleated erythrocytes from human and mammals. Erythrocyte membrane (EM) consists of lipid bilayer with intercalated integral proteins, and under-membrane network of peripheral proteins, mainly spectrin. Human EM contains three major proteins, spectrin (25-30 weight %), glycophorin (about 25 weight %), and the dimers and olygomers of the anion exchanger (about 25 weight %). Barrier function of the plasma membranes is vital property and the extent of its deterioration under adverse conditions reflects the sensitivity of cells to physical injury. At high temperatures cell membranes undergo permeabilization [3,4] whose midpoint temperature, T g , is frequently used as a measure for the thermal stability of cell membranes. It was found that a slight increase (1-2 o C) in T g is regularly accompanied by a significant (40-60 %) increase in the thermoresistence of various types of cells [5,6], including mammalian erythrocytes [7]. Thermal hemolysis is a temperature-activated process with activation energy (E a) of 300±15 kJ/mol [8,9,10,11]. Of all temperature-activated processes (excluding the dehydration of ions), similar value of E a is displayed only by the thermal denaturation of proteins (E a between 300-350 and 800 kJ/mol) and the conformation changes in proteins (E a <300 kJ/mol). Based on this value of E a it could be precluded that the rate-limiting step of thermal hemolysis includes a conformational transition in a protein or in a group of similar proteins. Ions flow mainly through the protein-mediated pathways of cell membranes and only a small portion of this flow is ascribed to the basal (residual) permeability, P. Despite its small value, P is enhanced at hyperthermia causing hemolysis. According to data obtained with human erythrocytes [12,13,14,15], the rise in P precedes thermal hemolysis, which demonstrates colloid-osmotic character in its initial stage. The increase in P, which underlies the hemolysis, was studied in various temperature intervals: 47-65°C [12], 46-54°C [13], 50-58°C [15] and 38-57°C [16; 17]. In each of these intervals P is a temperature-activated parameter (P = P o .exp(-E a /RT)) with the same E a of 250 ± 15 kJ/mol. This indicates a single mechanism for thermal activation of P, which serves as the prime target of heat in thermal hemolysis over the entire range of 38-60°C. For the 50-58°C interval, the target was identified with the heat-induced, cytosole-independent transition in EM at T g , which equals 61°C for human erythrocytes [14,15]. This transition enhances ion permeability eliciting colloid-osmotic lysis at constant temperatures [15] and during heating with constant rate [14]. The same mechanism of thermal hemolysis was found in mammalian erythrocytes [7,17]. As for human erythrocytes, the E a for the activation of P was again 250 ± 15 kJ/mol. The value of T g , however, demonstrated species differences in respect to the sphingomyelin content of EMs. The