The effect of myosin sulphydryl modification on the mechanics of fibre contraction

Glycerinated rabbit psoas fibres have been modified with paramagnetic probes ( IASL and MSL) which react selectively with the reactive sulphydryl on the myosin head. The extent of SH-1 modification was monitored by extracting myosin and measuring its ATPase activity in the presence of EDTA and of Ca2+. The isometric tension, stiffness, maximum velocity of contraction (slack test), and the force-velocity relation was measured as a function of the degree of SH-1 modification. Reaction of up to 50% of SH-1, i.e. 50% reduction in the K+-EDTA ATPase activity of extracted myosin, produced little change (less than 10%) in any of the fibre parameters. Modification of 75% of the SH-1 sites produced small decreases (15-30%) in the magnitude of all parameters, while reaction of more than 90% of SH-1 required long reaction times and produced decreases of 40-75%. In all cases the velocities of contraction decreased in parallel with the decrease in tension, while the decrease in stiffness was less pronounced. We conclude that a large fraction of muscle fibre SH-1 groups can be modified without greatly affecting the mechanical performance of the fibre. At least a portion of the decrease in fibre parameters that is observed at high levels of SH-1 modification can be attributed to modification of other sulphydryls by the probes. The reaction of both SH-1 and nonspecific sulphydryls abolishes myosin ATPase activity, and can account for approximately one half of the decrease in fibre parameters that is observed at high degrees of sulphydryl modification. We conclude that the modification of SH-1 does not greatly affect the function of a myosin head in the filament array of a fibre. This is in contrast to results obtained in vitro where SH-1 modification alters several rates in the interaction of myosin with ATP and decreases the actin-activated ATPase activity of myosin subfragments.