Yeast hexokinase inactivation by lead and its reversion by magnesium as one possible aspect of lead toxicity

Epidemiological studies suggest a positive association between blood lead levels and fasting blood glucose levels (BGL). The link between glucose homeostasis and Pb exposure is unclear. BGL is the result of multiple processes involving many enzymes. One of the critical initial steps in cellular glucose metabolism is its phosphorylation by ATP, catalyzed by hexokinase (HK) or glucokinase in hepatocytes. This phosphorylation prevents glucose inside cells from diffusing back out and commits glucose to further intracellular processing.

The aim of this study is to analyze the effect of lead and other divalent metals in hexokinase (HK) enzyme activity in vitro to understand the toxic effect of lead on a Mg²⁺-dependent enzyme. The highest HK activity was observed in the presence of Mg in absence of Pb.

Our results show that lead inhibits HK in a time-dependent fashion, thus requiring several minutes of preincubation with Pb but without glucose. A modified non-competitive inhibition model is the best to describe this loss of activity. Mg is the only metal that partially reverted the lead dependent loss of HK activity. These results suggest that lead forms stable metal complexes which interact with different protein targets, some of them displace for Mg and others available in protein conformation without glucose. Data calculated at the DFT level of theory show that all Metal-ATP complexes promote the glucose phosphorylation. Inhibition does not seem to depend on the formation of a PbATP complex.