Impact of High Pyruvate Concentration on Kinetics of Rabbit Muscle Lactate Dehydrogenase

Abstract

In order to evaluate the effectiveness of l-lactate dehydrogenase (LDH) from rabbit muscle as a regenerative catalyst of the biologically important cofactor nicotinamide adenine dinucleotide (NAD), the kinetics over broad concentrations were studied to develop a suitable kinetic rate expression. Despite robust literature describing the intricate complexations, the mammalian rabbit muscle LDH lacks a quantitative kinetic rate expression accounting for simultaneous inhibition parameters, specifically at high pyruvate concentrations. Product inhibition by l-lactate was observed to reduce activity at concentrations greater than 25 mM, while expected substrate inhibition by pyruvate was significant above 4.3 mM concentration. The combined effect of ternary and binary complexes of pyruvate and the coenzymes led to experimental rates as little as a third of expected activity. The convenience of the statistical software package JMP allowed for effective determination of experimental kinetic constants and simplification to a suitable rate expression:

$$ v = \frac{{{V_{max}}\left( {AB} \right)}}{{{K_{ia}}{K_b} + {K_b}A + {K_a}B + AB + \frac{P}{{{K_{I - Lac}}}} + \frac{{{B^2}A}}{{{K_{I - Pyr}}}} + \frac{{{B^2}Q}}{{{K_{I - Pyr - NAD}}}}}} $$

where the last three terms represent the inhibition complex terms for lactate, pyruvate, and pyruvate–NAD, respectively. The corresponding values of K _I–Lac, K _I–Pyr, and K _{I–Pyr–NAD} for rabbit muscle LDH are 487.33 mM⁻¹ and 29.91 mM and 97.47 mM at 22 °C and pH 7.8.