Nucleotide-induced changes in muscle fibres studied by DSC and TMDSC

Differential scanning calorimetry (DSC) was used in conventional and temper ature-modulated mode to study the energetics of myosin in skeletal muscle f ibres in different states of the actomyosin ATPase cycle. Psoas muscle fibr es from rabbit were used in the experiments with and without the presence o f nucleotides (ATP, ADP, AMP-PNP) and ATP or ADP + orthovanadate. In the complex DSC pattern, the higher transition referred to the head regi on of myosin. The enthalpy of the thermal unfolding depended on the nucleot ides, the conversion from a strongly attached state of myosin to actin to a weakly binding state was accompanied with an increase of the transition te mperature which was due to the change of the affinity of nucleotide binding to myosin. This was more pronounced in TMDSC mode, indicating that the str ong-binding state and rigor state differ energetically from each other. The different transition temperatures indicated alterations in the internal mi crostructure of myosin head region. The monoton decreasing TMDSC heat capac ities show that the C-p of biological samples should not be temperature ind ependent. (C) 2001 Elsevier Science B.V. All rights reserved.