A COMPARATIVE-STUDY OF THE THERMAL INACTIVATION OF CYTOSOLIC AND MITOCHONDRIAL ASPARTATE AMINOTRANSFERASES

Rates of irreversible thermal inactivation of cytosolic and mitochondr ial aspartate aminotransferases were measured over a large temperature range. Inactivation occurred by different kinetic pathways at high an d low temperature with a transition point at about 60 degrees C. This suggests that the isoenzymes exist in different conformations above an d below that temperature. Discontinuities in plots of ln(V-max) agains t 1/T provided confirmatory evidence for this hypothesis. Activation p arameters (Delta H double dagger and Delta S double dagger) for the th ermal inactivation process were calculated in the high and low tempera ture ranges. At high temperature the greater rate of inactivation of t he mitochondrial isoenzyme is determined largely by a high value of De lta S double dagger. This more than compensates for the fact that the Delta H double dagger is also greater for the mitochondrial isoenzyme indicative of greater intramolecular stabilising interactions compared with the cytosolic form. Thus the relative rates of inactivation are determined by the nature of the transition states rather than by intra molecular interactions in the folded proteins. At lower temperatures t he kinetic stabilities of the isoenzymes reverse with the mitochondria l isoenzyme inactivating more slowly. This is largely because of a con siderably smaller Delta S double dagger at low temperature which no lo nger compensates for the greater Delta H double dagger compared with t he cytosolic isoenzyme. (C) 1997 Elsevier Science B.V.