Effects of pH and Temperature on the Kinetics of Pyruvate Oxidation by Muscle Mitochondria from Rainbow Trout (Oncorhynchus mykiss)

We measured the kinetics of pyruvate oxidation by mitochondria of rainbow trout (Oncorhynchus mykiss) red muscle and the kinetics of pyruvate decarboxylation by partially purified pyruvate dehydrogenase (PDH) from the same tissue. The measurements were done at three temperatures (8°, 15°, and 22°C) and at two pH conditions, a stable pH and an adjusted pH, that is, a pH that mimics the cellular pH adjustments with body temperature (ΔpH/ΔT= -0.03°C⁻¹). At constant pH, a change in temperature from 15° to 8°C decreased the apparent Km of mitochondria for pyruvate. However, when pH covaried with temperature, the apparent Km did not change. The Q10 for the Vmax of pyruvate oxidation was 2.5 between 8° and 15°C and 0.89 between 15° and 22°C (adjusted pH). Temperature significantly affected the pyruvate kinetics of PDH when pH was maintained constant (7.8) but not when pH covaried with temperature. Thus, pH regulation would minimize the functional impact of changes in temperature for both PDH and mitochondria. Comparison of the pyruvate kinetics suggests that PDH partially determines the mitochondrial affinity for pyruvate as well as its sensitivity to temperature and pH. Between 8° and 15°C, the Q10 of the Vmax of PDH was lower than that of intact mitochondria, suggesting that PDH did not limit the maximal rates of mitochondrial pyruvate oxidation. The thermal sensitivity of mitochondrialpyruvate oxidation was lower than that of the aerobic scope of the trout.