MICROCALORIMETRIC STUDY ON THE INTERACTION OF F-ACTIN WITH MYOSIN ANDITS PROTEOLYTIC FRAGMENTS

The heat effect produced in the interaction of rabbit muscle F-actin w ith myosin and its proteolytic fragments (HMM, S-1) was measured with an MS-80 Calvet microcalorimeter (Setaram, France). An improved reacti on cell of the microcalorimeter was used. There were three thermokinet ic peaks in the measured curve of the interaction of myosin with F-act in in the absence of ATP, which were exothermic, endothermic and exoth ermic in order of appearance. When ATP was added to F-actin before mix ing with myosin, the curve remained similar in feature to that in the absence of ATP. But the first peak (exothermic) became smaller and the second and third peaks became larger. There was only one exothermic p eak in the measured curve of the interaction between S-1 and F-actin. But ATP caused big changes in the curve of their interaction. With add ition of ATP to F-actin solution before it was mixed with S-1, there w ere four thermokinetic peaks in the curve which were endothermic, exot hermic, endothermic, and exothermic in order of appearance. The measur ed curves of interaction between HMM and F-actin in the presence and a bsence of ATP were quite similar to that of interaction between S-1 an d F-actin.