pH-Temperature Dependence of Actomyosin Superprecipitation

ABSTRACT

The effect of temperature on the pH-dependence of actomyosin superprecipitation was studied, using actomyosin extracted from the rabbit and frog skeletal muscle tissues. The pH optima of superprecipitation was rather broad in both the rabbit and frog actomyosin. In the frog, superprecipitation measured at 16-42 degrees C was relatively independent of pH variations between 6.7 to 8.5, but it was significantly inhibited at pHs outside of this range, showing a sharp inflection of the curve. The pH at the inflection point was inversely proportional to the incubation temperature, but the (OH-)/(H+) ratio at the inflection point was not changed with temperature. The log (OH-)/(H+) was approximately -0.6 on the acidic side and 3.16 on the alkaline side. Similarly, superprecipitation of the frog actomyosin was virtually independent of the medium pH of the intermediate range (approximately 6.0-8.5); but it was drastically inhibited at pHs below or above this range, thus revealing a sharp inflection of the curve. Again, the pH at the inflection point changed inversely with temperature, preserving a constant (OH-)/(H+) ratio. The log (OH-)/(H+) ratio at the inflection point was approximately -2 on the acidic side and 3.5 on the alkaline side. The above pH effects were not associated with irreversible protein damage or with the changes in buffer species. These results strongly suggest that suppression of the superprecipitation of rabbit and frog actomyosin gels, at a low and high pH, be due to alterations in the fractional dissociation of histidine-imidazole and cysteine-SH groups, respectively.