Cardiac troponin possesses two high-affinity Ca2+-Mg2+ binding sites and one low-affinity Ca2+-specific site, distinguishing its regulatory mechanism from skeletal troponin.
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Cardiac troponin Ca2+ binding properties warrant no clinical change; leaves open translation of regulatory distinctions to human myocardium.
The cardiac troponin (Tn) complex, consisting of a Ca2+-binding subunit (TnC), an inhibitory subunit (TnI), and a tropomyosin-binding subunit (TnT), has been reconstituted from purified troponin subunits isolated from bovine heart muscle.The Ca2+-binding properties of cardiac Tn were determined by equilibrium dialysis using either EGTA or EDTA to regulate the free Ca2+ concentration.Cardiac Tn binds 3 mol Ca2'/ mol and contains two Ca2+-binding sites with a binding constant of 3 X 10' "' and one binding site with a binding constant of 2 X 10' M-'.In the presence of 4 m~ MgC12, the binding constant of the sites of higher a nity is reduced to 2 X lo7 "', while Ca2+ binding to the site at the lower affinity is unaffected.The two high affinity Ca2+-binding sites of cardiac Tn are analogous to the two Ca2+-Mg2+ sites of skeletal Tn, while the single low affinity site is similar to the two Ca2+-specific sites of skeletal Tn (Potter, J. D., and Gergely, J. (1975)
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Holroyde et al. (1980) studied this question.
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