Building a model of ammonium excretion in two species of marine zooplankton based on glutamate dehydrogenase kinetics

Abstract

NH4 + production in 2 species of marine zooplankton was predicted from glutamate dehydrogenase (GDH) activity using general bisubstrate enzyme kinetics during a period of starvation. In addition to starvation, we studied the effect of food quality on both NH4 + excretion rates (RNH4 +) and the biochemical composition of zooplankton. The mathematical function used here relies on Michaelis-Menten principles and assumes that the synthesis of NH4 + is controlled mainly by the maximum velocity of GDH, the intracellular pool of substrates and the kinetic constants. This simple model described reasonably well the shifts in RNH4 + that occurred within the time course of starvation in both groups of zooplankton. The calculated RNH4 +, however, underestimated the actual RNH4 +. Furthermore, we used single substrate kinetics to address the magnitude of either the allosteric inhibition or the activation by purine nucleotides. We found that allosterism is more important in healthy physiological conditions. Overall, the use of first-principles kinetics seems to provide better estimates of RNH4 + than applying a general GDH/RNH4 + ratio on the measurements of GDH; therefore, its application may be of potential interest when studying zooplankton RNH4 + in heterogeneous marine ecosystems.