Kinetic properties of myosin heavy chain isoforms in single fibers from human skeletal muscle

The head portion of the myosin heavy chain is essential in force generation . As previously shown, Ca2+-activated muscle fibers from rat and rabbit dis play a strong correlation between their myosin heavy chain isoform composit ion and the kinetics of stretch activation, corresponding to an order of ve locity: myosin heavy chain Ib > myosin heavy chain IId(x) > myosin heavy ch ain IIa >> myosin heavy chain I. Here, we show a similar correlation for hu man muscle fibers (myosin heavy chain IIb > myosin heavy chain IIa >> myosi n heavy chain I), suggesting isoform-specific differences between the kinet ics of force-generating power strokes. The kinetics of myosin heavy chain I are similar in human and rodents. This holds also true for myosin heavy ch ain IIa, but human myosin heavy chain IIb is slower than rodent myosin heav y chain IIb, It is similar to rodent myosin heavy chain IId(x), (C) 1999 Fe deration of European Biochemical Societies.