Mechanism of inhibition of lysyl hydroxylase activity by the organophosphates malathion and malaoxon

Direct inhibition of lysyl hydroxylase by malathion and malaoxon was observed in an in vitro enzyme assay with recombinant lysyl hydroxylase expressed via a baculoviral system. The IC50 values for malathion and malaoxon were estimated to be approximately 60 and 45 mM, respectively. Additional kinetic studies showed this inhibition to be competitive or partially competitive with respect to the synthetic (collagen) peptide, partially uncompetitive with respect to Fe²⁺, and partially noncompetitive with respect to ascorbic acid. The calculated values for the K_i were consistent with the IC50 values. Allosteric effects were not found for any of the cofactors tested, the peptide substrate, or the inhibitors. Interactions were found to be unimolecular for lysyl hydroxylase and its substrate and cofactors as well as for the inhibitors malathion and malaoxon. A computer search of a protein structure database showed an unexpected region of partial homology between the active site sequence of acetylcholinesterase and a segment of lysyl hydroxylase, suggesting a possible molecular basis for these observations. These results suggest the possibility of a novel and hitherto unexpected class of inhibitors of lysyl hydroxylase, based on the organophosphate structure, that might be of value for testing as antifibrotic drugs.