Influence of osmolytes, thin filaments, and solubility state on elasmobranch phosphofructokinase in vitro

Skeletal muscle phosphofructokinase (PFK) purified from the thornback ray is rapidly inactivated by urea concentrations as low as 50 mM at pH values below 7.0. Urea-induced loss of PFK activity is not offset by trimethylamine-N-oxide. Protection against urea-inactivation in vivo, where urea concentration may approach 0.5 M, may be due to two effects. Filamentous (F) actin and muscle thin filaments moderately reduce the urea-induced loss of PFK activity. The binding of PFK to F-actin and to thin filaments is shown by ultracentrifugation experiments. PFK activity in vivo also may be stabilized in this species by the formation of a particulate enzyme form which is totally resistant to inactivation by physiological concentrations of urea.