Effects of urea and trimethylamine‐N‐oxide on ATPase of requiem shark myofibril and its constituents

: The effects of trimethylamine-N-oxide (TMAO) on the urea-resistibility of requiem shark myofibrils were investigated, using Ca2+- and Mg2+-ATPase activities as a parameter. Both activities were hardly changed or activated up to 0.6 M urea. In contrast, the two activities both decreased to less than 50% in the presence of TMAO up to 0.5 M. When measured at a 2 : 1 molar ratio of urea and TMAO, Ca2+- and Mg2+-ATPase activities were similar to those in the presence of TMAO alone, indicating that TMAO reduced the urea-resistibility of myofibrils. Myosin, the most abundant protein in myofibrils, from requiem shark exhibited the effects of urea and TMAO on its Ca2+-ATPase activity, which was primarily similar to those of myofibrils. However, Ca2+-ATPase activities in the coexistence of urea and TMAO for actomyosin reconstituted from requiem shark myosin and chicken F-actin were approximately average of those measured independently in the presence of either urea or TMAO alone. Carp myofibrils, reconstituted actomyosin and myosin, which were used as teleost references, all showed a tendency in the effects of urea and TMAO on Ca2+-ATPase activities that was similar to those of requiem shark counterparts.