Kinetic measurement of esterase-mediated hydrolysis for methacrylate monomers used in dental composite biomaterials

Methacrylate-based monomers are routinely used in medical biomaterials. Monomers undergo polymerization reactions to form the solid resin. These polymerization reactions can be incomplete thus making unpolymerized monomer available for possible biodistribution. Understanding the fate of these monomers is essential not only for their toxicological profile but also for development of future biomaterials. Aromatic methacrylate-based monomers included in this study were bisphenol A dimethacrylate and bisphenol A diglycidyl dimethathacrylate; aliphatic methacrylate monomers were 2-hydroxyethyl methacrylate and triethyleneglycol dimethacrylate. These compounds contain ester moieties thought to be susceptible to esterase-mediated hydrolysis. The hypothesis was that the ester bond of the methacrylate monomers can be hydrolyzed by esterases and these reactions would occur in a measurable, time-dependent manner confirmed by specific Michaelis-Menten kinetic relationships. Including aliphatic and aromatic methacrylate monomers in this work allowed for structure-based comparisons. In vitro enzymolysis of the test compounds by acetylcholinesterase and cholesterol esterase was performed in buffered solutions. The hydrolysis reactions were monitored by high performance liquid chromatography with ultraviolet detection. The disappearance of parent compound and appearance of hydrolysis products were quantitated. The aromatic methacrylate monomers, bisphenol A dimethacrylate and bisphenol A diglycidyl dimethacrylate, were resistant to acetylcholine esterase hydrolysis but were converted by cholesterol esterase. The putative xenoestrogen, bisphenol A, was identified as a hydrolysis product from bisphenol A dimethacrylate conversion. Cholesterol esterase induced hydrolysis of bisphenol A diglycidyl dimethacrylate yielded a K_m value of 1584 μM and V_max of 14 μM min−1. Triethyleneglycol was converted by both esterases with calculated K_m values of 394 and 1311 μM for acetylcholine esterase and cholesterol esterase, respectively; V_max values were 3.8 and 3.9 μM min−1 for acetylcholine esterase and cholesterol esterase, respectively. The water-soluble compound 2-hydroxyethyl methacrylate was resistant to hydrolysis by esterases. Aliphatic methacrylate monomers of short chain length in their ester substituent bearing hydroxyl groups are resistant to esterase actions. Aromatic methacrylates are resistant to acetylcholine esterase hydrolysis but susceptible to cholesterol esterase hydrolysis; longer side chains on the phenyl ring impart more resistance. Structure limitations in esterase-mediated hydrolysis of methacrylates were identified from the results.