Light chains from slow-twitch muscle myosin.

Myosin light chains have been isolated from slow-twitch soleus muscles of rabbit and cat. Two chemically related light chains of molecular weight about 22 000 have been identified from their thiol sequences in each species, and these have been further characterized by amino acid analysis and peptide mapping studies. These light chains are related to the alkali light chains of rabbit fast-twitch muscles and to the larger cardiac light chain from bovine heart muscle. The presence of two chemically related but phenotypically distinct light chains within single muscles suggests the presence of myosin isoenzyme or that the myosin molecule has a different light chain associated with each subfragment-1 head. Although the stoichiometry of these two light chains had not been determined, the former of these two conclusions is favoured by analogy with experiments on fast-twitch myosins. In addition to these related light chains, soleus muscle myosin, like fast-twitch myosins, contains a third light chain of about 19 000 molecular weight. Unlike the corresponding light chain of rabbit fast-twitch myosin, this 19 000-Mr light chain contrains no cysteine residues. The distribution of thiol peptides together with the characteristic mobilities of these light chains on polyacrylamide gels in the presence of sodium dodecyl sulphate provides a 'fingerprint' of myosins from different muscle types. This has been used to look for the presence of fast-twitch myosin in soleus muscle, the results showing both rabbit and cat soleus muscles are homogeneous in their myosin type within the sensitivity of detection of the methods. These techniques have also been used to confirm the reciprocal transformation of light chains in myosins isolated from cross-reinnervated muscles. Finally the relationships between these criteria for different myosin types and histochemical procedures for muscle fibre typing are discussed.