Effects of N-ethylmaleimide Modification on ATPase Activities of Fish Myosin and Myosin B

Abstract

The Ca²⁺ -ATPase activity of fish myosin was activated by N-ethylmaleimide (NEM) treatment, while the EDTA-ATPase activity was inhibited. These changes indicate that fish myosin, like rabbit myosin, contains first reactive thiol groups (SH₁).
On the other hand, myosin SH₁ in myosin B system was completely unreactive with NEM under the conditions where the association of actin-myosin occurred, i.e., at physiological ionic strength and pH, and in the absence of ATP. These results suggest that myosin SH₁ in fish muscle may be masked with actin under the conditions of postmortem storage. The Mg²⁺ -ATPase activtity of myosin B, however, increased by NEM treatment under the conditions where SH₁ was unreactive.
It was also found that the thermal stability of the ATPase activity of fish myosin B. on storage at high ionic strength, was decreased remarkably by the NEM treatment. This thermal instability of the NEM treated myosin B may be due to the dissociation of myosin and actin.