A study of the biochemical properties of native vertebrate skeletal myosin filaments.

Chymotryptic S-1 lacking in DTNB LC can be obtained from synthetic rabbit myosin filaments (Weeds and Taylor, 1975; Weeds and Pope, 1977). I have now shown however that chymotryptic digestion of 2H2O-H2O gradients-purified native myosin filaments (Emes and Rowe, 1978) yields S-1 containing the DTNB LC. Our results strongly suggest that this difference between synthetic and native myosin filaments upon subjection to chymotryptic digestion is mostly due to a difference in the myosin conformation within the two types of filaments, since chymotryptic S-1 from filaments formed from solubilised native filaments produced conventional S-1. Detailed studies of the ATPase activities of native filaments and the derived S-1 (before and after exposure to high ionic strength) have been performed using an isotopic assay (?-32P ATP). Evidence has been obtained to indicate that DTNB LC before exposure to high ionic strength possesses the potential to repress ATPase activities and it is believed that native DTNB LC plays a role in maintaining the low ATPase activities in relaxed muscle. ATPase activities of rabbit skeletal myofibrils have been shown to be calcium-sensitive at physiological ionic strength (Lehman, 1977; 1978). Using purified and relaxed filaments, I have shown that these preparations have calcium-sensitive ATPase activities at physiological ionic strength and also at low ionic strengths in the case of purified filaments. My results suggest that DTNB LC before "seeing" high ionic strength is responsible for calcium-sensitivity. Evidence is presented to suggest that there are at least two populations of myosin heavy chains.