Isolation of myosin from the obliquely striated muscle of squid mantle was studied by comparing the various conventional isolation methods used for the vertebrate and invertebrate muscles. A method suitable to the squid myosin is proposed. The purity of each obtained preparation was checked by SDS polyacrylamide gel electrophoresis. The myosin preparations obtained by the conventional methods did not give any band corresponding to the myosin heavy chain, but those less than 14×10⁴ in molecular weight were not free from contaminating proteins such as paramyosin and actin.
_{The myosin preparation obtained by the proposed method showed an electrophoretic profile with a heavy chain band (18-20×10⁴}) and two light chain bands (1.2 and 1.5×10⁴) of myosin with a faint band of paramyosin (10×10⁴) in 7.5% gel.
The tryptic digestion of the squid myosin shows that it is less resistant than fish myosins, and much less than rabbit myosin. The difficulties in obtaining an intact heavy chain during the isolation procedures were attributed to the fragility of the squid myosin molecule and to the possibility of high catheptic activity in the squid muscle.