Chicken gizzard myosin

The Ca ++-ATPase activity of chicken gizzard myosin is increased 2- to 3-fold by tryptic treatment and by various substances, urea, guanidine-HCl, sodium dodecylsulfate, KCNS, LiBr, ethanol, or ethylene glycol. These substances also activate the Ca ++-ATPase of rabbit uterus myosin but have no effect on the Ca ++-ATPase of chicken breast myosin. Maximal Ca ++-ATPase activity of gizzard myosin is observed when 38 free NH 2-terminal groups in 600,000 gm of this myosin are liberated by trypsin. At this stage most of the myosin remains intact in the analytical ultracentrifuge. The addition of 1.0–2.0 M urea to gizzard myosin, pH 7.0, results in a 3-fold decrease in the Michaelis constant. The effect of urea on the activation of Ca ++-ATPase of gizzard myosin is reversible. Twelve cysteine residues per 600,000 gm of gizzard myosin are reacting with iodoacetate in the presence of 1.5 M urea in contrast to the nonreactivity of the SH groups without urea. Gizzard myosin retains all of its Ca ++-ATPase activity after 12 of its SH groups have been alkylated. From comparative experiments with iodoacetate and iodoacetamide it is concluded that the ATPase site of gizzard myosin undergoes an alteration in the presence of urea as a result of conformational changes in regions of the myosin molecule removed from the ATPase site. The Ca ++-ATPase and NTPase activities of gizzard myosin are much lower than those of breast myosin even in the presence of modifiers. However, the ability of the two different types of myosin to combine with rabbit skeletal F-actin is the same. Amino acid analyses show differences in the composition of gizzard and breast myosin. Gizzard myosin contains cystine and has a slightly lower cysteine content than does breast myosin.