HARTZLER, L.K.*; HICKS, J.W.; BENNETT, A.F.; STINNER, J.N.: Role of Temperature and pH on Enzyme Activity in Air-breathing Ectotherms
Maintenance of the integrity of functional groups in enzymes has been proposed as a key reason for regulation of acid-base changes during changes in body temperature. As pH of tissue changes, different titratable groups are affected in enzymes or in their super-enzyme complexes. Three major questions are addressed in this study: (1) Do different enzymes have the same “optimal” pH? (2) Do animals selectively regulate pH at a level to maximize (preserve) enzyme activity? (3) Is enzyme sensitivity to pH species specific? To address these questions, the activity of four enzymes (Citrate Synthase, 3-Hydroxyacyl CoA Dehydrogenase, Lactate Dehydrogenase, and Pyruvate Kinase) were measured in Bufo marinus, Rana catesbeiana, and Coluber constrictor skeletal muscle at 30oC, 20oC, and 10oC over a range of pH from 6.0 to 8.0 Units. Results from this experiment suggest that maximal activity for each of these enzymes occurs at a different pH so that there is no one optimal pH for enzyme activity; therefore, in vivo pH does not correspond to maximal activity. While intracellular pH changes differently for these three species (B. marinus, -0.005 U oC-1; R. catesbeiana, -0.014 U oC-1; C. constrictor, -0.009 U oC-1), they appear to share the same enzyme sensitivity to pH. These results suggest that activity of enzymes is fairly constant over a range of pH broader than previous hypotheses would predict. These results are not consistent with the alpha-stat, z-stat, and relative alkalinity hypotheses. NSF IBN 0091308.
