Two recent studies have reported that polyanions can affect protein stability, but it is unclear whether these stability effects are protein-specific or whether more general trends or influences are operative.  Although polyanions are pervasive in the intracellular milieu, there have been no studies examining the effects of polyanions on enzyme activity.  These polyanion effects are distinct from both general ionic strength effects and the effects of binding to specific high affinity binding sites and appear to be operative in the millimolar concentration range.  This project aims to determine the effect of the physiologically relevant polyanion ATP and the model polyanion sodium polyacetate (NaPAc) on model enzymes.  The mammalian enzymes malate dehydrogenase (MDH) and two isozymes of lactate dehydrogenase (LDH-1 and LDH-5) were selected for this study.  In addition to catalyzing similar NADH-dependent redox reactions, these enzymes also exhibit substrate inhibition at high concentrations of substrate (oxaloacetate for MDH and pyruvate for LDH).  In MDH, substrate inhibition is sensitive to both the type and concentration of polyanion.  Millimolar concentrations of NaPAc cause an increase in substrate inhibition, whereas ATP at comparable concentrations results in a decrease in substrate inhibition.  In contrast, initial experiments with LDH-1 suggest that this enzyme is insensitive to similar concentrations of ATP.  The results of a detailed comparative study of the effects of polyanions on these three dehydrogenase enzymes will be presented, but our initial data suggests that this polyanion effect may reflect a previously unrecognized added level of metabolic regulation.

Polyanion effects on the kinetic behavior of mammalian dehydrogenase enzymes

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