Misconceptions regarding basic thermodynamics and enzyme kinetics have led to erroneous conclusions regarding the metabolic importance of lactate dehydrogenase isoenzyme expression

TY - JOUR

T1 - Misconceptions regarding basic thermodynamics and enzyme kinetics have led to erroneous conclusions regarding the metabolic importance of lactate dehydrogenase isoenzyme expression

AU - Bak, Lasse K

AU - Schousboe, Arne

N1 - © 2017 Wiley Periodicals, Inc.

PY - 2017/11

Y1 - 2017/11

N2 - Lactate dehydrogenase (LDH) catalyzes the interconversion of pyruvate and lactate involving the coenzyme NAD+. Part of the foundation for the proposed shuttling of lactate from astrocytes to neurons during brain activation is the differential distribution of LDH isoenzymes between the two cell types. In this short review, we outline the basic kinetic properties of the LDH isoenzymes expressed in neurons and astrocytes, and argue that the distribution of LDH isoenzymes does not in any way govern directional flow of lactate between the two cellular compartments. The two main points are as follows. First, in line with the general concept of chemical catalysis, enzymes do not influence the thermodynamic equilibrium of a chemical reaction but merely the speed at which equilibrium is obtained. Thus, differential distribution of LDH isoenzymes with different kinetic parameters does not predict which cells are producing and which are consuming lactate. Second, the thermodynamic equilibrium of the reaction is toward the reduced substrate (i.e., lactate), which is reflected in the concentrations measured in brain tissue, suggesting that the reaction is at near-equilibrium at steady state. To conclude, the cellular distribution of LDH isoenzymes is of little if any consequence in determining any directional flow of lactate between neurons and astrocytes.

AB - Lactate dehydrogenase (LDH) catalyzes the interconversion of pyruvate and lactate involving the coenzyme NAD+. Part of the foundation for the proposed shuttling of lactate from astrocytes to neurons during brain activation is the differential distribution of LDH isoenzymes between the two cell types. In this short review, we outline the basic kinetic properties of the LDH isoenzymes expressed in neurons and astrocytes, and argue that the distribution of LDH isoenzymes does not in any way govern directional flow of lactate between the two cellular compartments. The two main points are as follows. First, in line with the general concept of chemical catalysis, enzymes do not influence the thermodynamic equilibrium of a chemical reaction but merely the speed at which equilibrium is obtained. Thus, differential distribution of LDH isoenzymes with different kinetic parameters does not predict which cells are producing and which are consuming lactate. Second, the thermodynamic equilibrium of the reaction is toward the reduced substrate (i.e., lactate), which is reflected in the concentrations measured in brain tissue, suggesting that the reaction is at near-equilibrium at steady state. To conclude, the cellular distribution of LDH isoenzymes is of little if any consequence in determining any directional flow of lactate between neurons and astrocytes.

KW - Journal Article

KW - Review

U2 - 10.1002/jnr.23994

DO - 10.1002/jnr.23994

M3 - Review

C2 - 28150419

SN - 0360-4012

VL - 95

SP - 2098

EP - 2102

JO - Journal of Neuroscience Research

JF - Journal of Neuroscience Research

IS - 11

ER -