Abstract
A paper describing increased brain lactate concentration with age recently caught our attension [1]. The proposed explanation for the increased brain lactate with age was a shift in the brain lactate dehydrogenase (LDH) isoenzyme pattern, which also occurred with age. While the observations are highly interesting, we found the explanation unlikely under steady state conditions, since LDH is regarded a near-equilibrium reaction and since the equilibrium constant of all the LDH isoenzymes, of course, are the same [2]. We therefore decided to evaluate the question further, including a brief historical review, since a quick examination of common textbooks of biochemistry on the LDH isoenzyme question suggests that it is in fact common place to confer significant physiological importance to the different kinetic properties of the isoforms of LDH [3-9]
Highlights
A paper describing increased brain lactate concentration with age recently caught our attension [1]
No.5 pyruvate [1,3,4,5,6,7,8,9] in spite of changed kinetic constants the equilibrium constant, Keq, is the same for all isoenzymes since it is the same chemical reaction being catalyzed. This is stated in the Haldane Equation, relating Keq with the kinetic constants of the forward and reverse reaction: Keq = (Vmax f Kmp)/(Vmax r Kms) the total lactate dehydrogenase (LDH) activity is high in most tissues compared to metabolic flux and the reaction likely to be close to equilibrium under steady state conditions
We propose that under such conditions there may be significant physiological effect of the isoenzyme pattern of a given tissue
Summary
A paper describing increased brain lactate concentration with age recently caught our attension [1]. In the 1960’s it was demonstrated that the LDH isoenzymes showed different kinetic properties with respect to substrate affinity and inhibition, where the M-dominated forms have a 3.5 – 7 times higher Km value for pyruvate than H-dominated. The H-dominated isoenzymes have a lower Km value for pyruvate and lactate than M-dominated forms and are inhibited by pyruvate concentrations in the physiological range (Table 1).
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