Introduction Actin is a highly conservative protein occurring in high abundance in all eukaryotes playing central role in the function of living systems. Apart from its essen- tial role in the muscle contraction it can also be found as a part of the cytoskeleton, as it is the major compo- nent of the microfilament system. The cytoskeleton has central role in the life of the living cells as it is in- volved in the intracellular traffic [1], the endo- and exocytosis [2] and cell movements [3] as well. The two forms of the actin within the cells are the monomer (globular or G-actin) and polymer (fila- mentous or F-actin) forms. Six isoforms of actin can be distinguished in the nature based on their isoelec- tric points (three a-isoforms: pI=5.40; one a-isoform: pI=5.42; two a-isoforms: pI=5.44) [4]. The b- and g-isoforms can be found mainly in non-muscle cells [4]. The a-isoforms are found mainly in the muscle tissues and can differ only in a few of their amino acid residues. It is hypothesised that each of the muscle isoactin is specially adapted to the function of its re- spective tissue and the minor variations among them have developmental and/or physiological relevance [5]. The different isoforms of actin with different ori- gins can differ in their thermodynamic stability [6, 7] and/or polymerisation properties as well [8–11]. In a special case of heart diseases the a-skeletal actin accumulated while the occurrence of the a-car- diac isoforms decreased subsequently [12]. This find- ing can indicate the importance of the tissue specific presence of the actin isoforms [13]. The actin can be sensitive to the physico-chemi- cal properties of the environment [14, 15]. Its dyna- mic and structural properties can be influenced by the bound divalent cations [14–19], the pH of the sur- roundings [20], the shifting from monomer to its fila- mentous forms [21] and its binding of different lig- ands [22–26] as well. The differential scanning calorimetry is a powerful biophysical tool to characterise the thermal properties of proteins [27–31]. It is widely used in the measurements of the calorimetric features of the muscle actin and its associates as well [32–34]. The measured thermal pa- rameters can inform us about the thermodynamic prop- erties of a protein [35–39]. One of the important infor- mation from the heat absorption curves is the tempera- ture point where the change of enthalpy gets to its maxi- mum (T m ). This temperature can be correlated to the thermal stability of the protein as higher T m value can represent a more stable protein structure [25, 26]. The calculated values of free enthalpy changes can inform us about the stability of the protein (a higher value of the free enthalpy change can be correlated with a more sta- ble protein structure). Materials and methods Actin preparation Both the cardiac and skeletal actin was prepared from acetone-dried muscle powder prepared by the method of Feuer et al. [40]. The method of Spudich and Watt [41] 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2005 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 82 (2005) 287–290 THERMODYNAMIC CHARACTERIZATION OF DIFFERENT ACTIN ISOFORMS J. Orbán, Sz. Halasi, G. Papp, Szilvia Barkó and Beáta Bugyi * University of Pécs, Faculty of Medicine, Department of Biophysics, Pécs, Szigeti str. 12, 7624, Hungary The thermodynamic properties of the cardiac and skeletal a-actin isoforms were studied to characterise the molecular bases of the functional differences between them with the method of differential scanning calorimetry (DSC). The thermal properties of the actin filaments were described in the presence of calcium and magnesium ions as well. Based on the calculated free energy changes the a-cardiac actin filaments appeared to be more stable in its physiologically more relevant, magnesium saturated form. The mag- nesium saturated form of the a-cardiac actin filaments seemed to be more stable compared to the calcium saturated form of it. The enthalpy and entropy changes could differentiate between the a-cardiac and a-skeletal actin isoforms and between the calcium and magnesium saturated cardiac actin isoforms as well. Our results can demonstrate that the few differences between the amino acid sequences of the a-actin isoforms have an influ- ence on the thermal properties and maybe on the function of these proteins as well. Keywords: calorimetry, cardiac actin, divalent cation, skeletal actin, thermodynamics * Author for correspondence: beata.bugyi@aok.pte.hu