Adenosine triphosphate suppresses urea-induced denaturation of shark myosin calcium adenosine triphosphatase

Elasmobranchs, such as sharks, retain large amounts of urea in their body, but shark muscle proteins are not denatured by urea in vivo. There are two hypotheses regarding biosystems in the quenching of urea. One is that the accumulation of large amounts of trimethylamine-N-oxide suppresses protein denaturation by urea, and the other is that shark muscle proteins have an intrinsic resistance to urea. We focused on a protective effect of adenosine triphosphate (ATP) on fish muscle proteins, and studied the suppressive effect of ATP on urea denaturation of myosin Ca-ATPase from the scalloped hammerhead shark, Sphyrna lewini. Regarding myosin incubated with both ATP and urea, Ca-ATPase activity was higher than for that incubated with urea. The inactivation of Ca-ATPase activity induced by urea in the presence of ATP proceeded in two phases: a slow first-order reaction in an early phase and a fast first-order reaction in a late phase. The late phase seemed to start at the time when the remaining ATP had been depleted in the medium. These results suggest that ATP suppressed the denaturation of myosin Ca-ATPase by urea. ATP may act in vivo as a safeguard against muscle protein denaturation by urea.