Microheterogeneity around the reactive lysine residue in the myosin heavy chain from rabbit skeletal muscle

A molecule of myosin subfragment 1 (S1) from rabbit skeletal muscle has one highly reactive lysine residue, Lys-83 (RLR), in the heavy chain, which is rapidly and stoichiometrically modified by trinitrobenzenesulfonate. Our previous kinetic study (Komatsu, H., Emoto, Y., and Tawada K. (1993) J. Biol. Chem. 268, 7799-7808) showed that although MgPPi reduces the maximum number of trinitrophenylated RLR down to about 0.5 mol/mol of S1, this half-stoichiometric modification of RLR is not the result of any heterogeneity in the primary structure of S1. However, this result conflicts with a previous report in which the half-stoichiometric trinitrophenylation has been reported to be related to Pro/Ser microheterogeneity at the 78th residue position in the heavy chain of rabbit skeletal muscle myosin. To resolve the conflict, we isolated peptides containing both the 78th residue and RLR from the two different preparations of S1, one whose RLR was trinitrophenylated in the presence of MgPPi and the other whose RLR was not trinitrophenylated in the presence of MgPPi, and then we determined the amino acid sequences of the peptides. We found the same Pro/Ser microheterogeneity at the 78th position in the peptides from these S1s and thus concluded that this microheterogeneity has no correlation to the half-stoichiometric trinitrophenylation of RLR.