Troponin-I is one of three subunits that form the troponin complex of the thin filaments of striated muscle. The troponin complex serves as a calcium-sensitive switch that regulates striated muscle contraction. Tn-I binds actin and inhibits actomyosin ATPase activity in the abscence of calcium. Skeletal muscle Tn-I exists in two isoforms, TnI-fast and TnI-slow, expressed respectively in fast and slow-twitch muscle fibers. The two isoforms are characterized by molecular weights of ~21 kDa (fast) and ~22 kDa (slow). The ratios of slow and fast isoforms vary with muscle source and species. Skeletal muscle Tn-I offered by TriChem Resources is likely a mixture of slow and fast isoforms. It is purified by ion-exchange chromatography in conjunction with calcium dependent affinity chromatography on Troponin-C agarose.
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