CHARACTERIZATION OF THE SINGLE TYROSINE-CONTAINING TROPONIN-C FROM LUNGFISH WHITE MUSCLE, COMPARISON WITH SEVERAL FAST SKELETAL-MUSCLE TROPONIN CS FROM FISH SPECIES

Troponin C molecules from fast skeletal muscle of the following fish s pecies (trout, whiting, lungfish, tilapia, and cod) have been purified to homogeneity. Upon binding of CaZ+ or Mg2+, lungfish troponin C is the only troponin C from fish white muscle to show the typical increas e of tyrosine fluorescence emission quantum yield reported for rabbit fast skeletal muscle troponin C. The increase of tyrosine fluorescence signal occurring upon Ca2+ and Mg2+ titration of lungfish troponin C has been used to determine the corresponding affinity constants. With K-(CA) = 7.0 10(7) M-1 and K-(Mg) = 3.6 10(3) M-1, the sites probed by the tyrosine residue of lungfish troponin C are typical of the COOH-t erminal domain of fast skeletal troponin C's. The amino acid sequencin g of the tyrosine containing tryptic peptides has allowed us to positi on the single tyrosine residue at position 7 in the Ca2+ binding loop of the third site, in identical position to Tyr109 of troponin C from rabbit fast skeletal muscle. Metal ion binding studies followed by int rinsic fluorescence or Tb3+ luminescence indicate that the conformatio n of the structural domain of lungfish troponin C with one metal ion b ound is close to the physiological conformation of this domain. (C) 19 97 Elsevier Science Inc.