Solaro R J
J Supramol Struct. 1975;3(4):368-75. doi: 10.1002/jss.400030409.
In 2 mM MgATP, 0.08 ionic strength and 1 mM free Mg++ cardiac myofibrils bound 3.5 nmoles Ca/mg protein at maximal ATPase activation. Significant amounts of Ca were also bound to cardiac myosin with these same conditions. By subtraction of this myosin-bound Ca we obtained an estimate of 4 moles Ca bound per mole of myofibrillar troponin at maximal ATPase. We found, however, that Ca activation of myofibrillar ATPase could be estimated assuming that only two of troponin's Ca-binding sites are engaged in regulation of crossbridge activity. Increases in MgMTP from 0.3 to 5.0 mM raised the free Ca, giving half-maximal isomteric tension or ATPase. Although part of this shift is most probably due to changes in the number of rigor (nucleotide-free) actin-myosin linkages, the rightward shift of the free Ca++-activation relation with increase in MgATP from 2 to 5 mM appears to be due to effects of active (nucleotide-containing) actin-myosin linkages.
在2 mM的MgATP、0.08的离子强度和1 mM游离Mg++条件下，心肌肌原纤维在最大ATP酶激活时每毫克蛋白质结合3.5纳摩尔钙。在相同条件下，大量的钙也与心肌肌球蛋白结合。通过减去与肌球蛋白结合的钙，我们估计在最大ATP酶时每摩尔肌原纤维肌钙蛋白结合4摩尔钙。然而，我们发现，假设肌钙蛋白只有两个钙结合位点参与调节横桥活性，就可以估计肌原纤维ATP酶的钙激活情况。MgMTP从0.3 mM增加到5.0 mM会提高游离钙水平，使等长张力或ATP酶达到半最大水平。虽然这种变化部分可能是由于强直（无核苷酸）肌动蛋白-肌球蛋白连接数量的改变，但随着MgATP从2 mM增加到5 mM，游离钙激活关系向右移动似乎是由于活性（含核苷酸）肌动蛋白-肌球蛋白连接的影响。
