Myosin heavy chain isoform composition influences the susceptibility of actin-activated S1 ATPase and myofibrillar ATPase to pH inactivation

The objectives of this study were to determine the influence of pH and MyHC isoforms on myofibrillar and actin-activated myosin subfragment 1 (S1) ATPase activity and the protective effect of actin. Red (RST) semitendinosus and white (WST) semitendinosus myofibrils were incubated at pH 7, 6, or 5.5 with 0 or 2 mM ATP. RST and WST S1 isolates were incubated at pH 7, 6, or 5.5 in the presence or absence of actin. Maximum calcium-activated myofibrillar and actin-activated S1-ATPase activity were then assayed at pH 7. Incubation of myofibrils with ATP caused ATPase activity of myofibrils to decrease (p < 0.05) with the pH of the incubation. RST myofibrils maintained a higher (p < 0.0001) relative activity than WST myofibrils after incubation at pH 6 with ATP. Myofibrils incubated without ATP exhibited higher (p < 0.001) activities than those incubated with ATP following pH 5.5 treatments. WST myofibrils had a lower (p < 0.05) relative activity than RST following incubation at pH 5.5 without ATP. S1 ATPase activities decreased (p < 0.05) with incubation pH in WST samples, but not in RST samples. WST S1 activity was higher (p < 0.01) in samples exposed to pH 6 and 5.5 with actin bound compared to those incubated without actin. RST S1 exhibited a higher (p < 0.01) relative activity than WST samples following pH 5.5 treatment with bound actin. These data show that low pH inactivates myofibrils by altering actin-activated S1 ATPase. Furthermore, these results suggest that muscles with high proportions of fast fibers are more susceptible to pH inactivation of ATPase activity and that the protective effect of actin binding to myosin is less in fast fibers.