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 22000 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 fasttwitch myosins. In addition to these related light chains, soleus muscle myosin, like fast‐twitch myosins, contains a third light chain of about 19000 molecular weight. Unlike the corresponding light chain of rabbit fast‐twitch myosin, this 19000‐Mr light chain contrains no cysteine residues.The distribution of thiol peptides t