Kinetic studies on human lactate dehydrogenase isoenzyme-catalyzed lactate-to-pyruvate reaction |
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Authors: | Manchery P Menon Frissell R Hunter and Sheryl Miller |
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Institution: | (1) School of Sciences and Technology, Savannah State College, 31404 Savannah, Georgia, USA |
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Abstract: | In order to evaluate the functional differences that may exist in human lactate dehydrogenase (LDH) isoenzymes widely used
for clinical examination the kinetic and thermodynamic properties of the lactate to pyruvate reaction that they catalize were
examined. Small but significant differences in the kinetic properties of the three isoenzymes were observed. The difference
in the rate constants might affect the activity measurement of the individual isoenzyme as the initial velocity for the L-P
reaction catalyzed will not be the same for an equal amount of enzyme. Equilibrium constants for the overall reaction in the
presence and absence of pyruvate have been determined. On the basis of transition-state theory, the standard enthalpy and
free-energy changes for formation of ternary activated complex were positive, while the standard entropy change was negative.
Although the standard free-energy change was the same for activation by the three isoenzymes, the enthalpy and entropy changes
for the LDH-3-catalyzed reaction were different from the respective values for others. A large positive value for the free-energy
change and a negative value for the entropy change indicated unfavorable production of the activated complex (K
infeq.
sup╪
=1.89×10-16). The enzyme appears to stabilize and retain the activated complex until it dissociates into the products. |
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Keywords: | lactate dehydrogenase isoenzymes lactate-to-pyruvate reaction kinetics |
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