抄録
Single sarcomeres and myofibrillar fragments were isolated from pectoral muscle of chicken and the properties of their contraction induced by ATP were measured mainly in 0.04 M KCl at pH 7.0 and 25°C, in the presence of different concentrations of ATP, MgCl2, CaCl2, EDTA and EGTA. The following results were obtained.
1. The changes in the band patterns of myofibrillar fragments during contraction in-duced by ATP were as follows: in the first stage, a dense band appeared at the outer edge of the original H-zone. This band formation was accompanied by a gradual disappearance of the H-zone. In the second stage, the H-zone disappeared completely, the I-band shortened considerably and a dense band developed in the mid region of the sarcomere. Then, the so-called contraction band was formed at the Z-line and bands of lower density appeared at the outer edges of the original H-zone. At the next stage the latter bands fused, and then disappeared.
2. The minimum concentration of ATP required for contraction of myofibrillar frag-ments decreased and approached a constant value on increasing the concentration of MgCl2. Contraction was unaffected by addi-tion of CaCl2 or by treatment with EGTA. Contraction was inhibited by EDTA, and the concentration of this necessary for inhibition increased with increasing MgCl2 concentration.
3. The rate of ATP-hydrolysis depended only on the concentration of MgATP2-. It increased in proportion to the concentration of MgATP2- at comparatively low concentra-tions, i.e., lower than about 10 μM, and ap-proached a constant value of 150-200 μmoles Pi/g./min when the concentration of MgATP2-was sufficiently high.
4. On contraction of sarcomeres induced by a sufficiently high concentration of MgATP2-, the intensity of light-scattering of a suspension of myofibrillar fragments de-creased to about 55 per cent of its original value the and degree of decrease was independent of the angle of scattered light. When the concentration of MgATP2- was comparatively high, the rate of decrease in light-scattering intensity was observed to be in good propor-tion to the ATPase activity.
5. The average rate (Vs) of the initial stage of shortening of sarcomeres was measured under a microscope from the time-course of the increase in the number of contracted myofibrillar fragments. The relation of Vs to the rate of ATP-hydrolysis (Vh) was found to follow the equation, Vs=CVh/(1+Kc/[S])2, where Kc, is a constant, [S] is the concentra-tion of MgATP2- and C is the “coupling” coefficient.
6. The average amount of hydrolysis of ATP necessary to cause contraction of myo-fibrillar fragments to the stage of disap-pearance of I-bands and formation of faint contraction bands was found to be 5.6-7.2μmoles Pi/g. of protein. This amount was independent of the temperature.
