Microbial Esterase and the Degradation of Plasticizers

Plastic materials are playing an increasingly important role in our everyday lives. As a consequence, they are more frequently exposed to the environment; and the impact they or their additives might have on the latter becomes an important concern. Plasticizers are the most common additives found in plastics. They are used to improve on their physical properties (e.g. flexibility, workability). They are frequently found in PVC, where they can make up to 67% of the total weight of plastics [1]. They are encountered in a wide variety of products including building materials, blood bags, toys, cosmetics, inks, insect repellent and electrical wiring insulation [2-5]. Di-ester plasticizers represent the most common class of plasticizers. Of those, di-ester phthalates, in particular di-2-ethylhexyl phthalate (DEHP), are the most widely produced and used [1]. Plasticizers are not covalently bonded to the plastic polymer matrix and, consequently, have the ability to leach out of the material. This, along with the omnipresence of plastic materials, explains why plasticizers have been found in landfill leachates [6, 7], open waters [4, 8-10], food [11], drinking water, precipitations and soils [9]. In fact, Ribbons et al. stated that DEHP was found in 42% of the environmental samples where it was sought [12]. Plasticizers are therefore considered ubiquitous contaminants of aqueous and soil environments. In fact, the extent of the environmental release has lead to the inclusion of six phthalates, including DEHP, in the United States Environmental Protection Agency list of priority pollutants [13]. Due to their stability, biodegradation is the most likely means of plasticizer degradation in the environment. Studies conducted with pure strains [12, 14-16] or consortia [17, 18] of microorganisms have shown that di-ester plasticizers could be readily degraded by many organisms under a wide array of conditions. However, some recent studies have shown that the degradation of compounds such as DEHP and di-2-ethylhexyl adipate (DEHA) was only partial and that the resulting metabolites were recalcitrant and increased the toxicity of the aqueous system [16, 19, 20]. These metabolites have been found in diverse environmental samples [9]. It then becomes important to understand the factors influencing the degradation mechanisms of plasticizers and their long term impact on the environment. The first two steps in the biodegradation of di-ester plasticizers involve the hydrolysis of the ester bonds by esterase or lipase enzymes. Previous studies have isolated the esterases involved in the hydrolysis of di-ester phthalates and adipates from different microorganisms [21-23]. However, these studies were concentrating on the characterization of the enzymes and not on the factors affecting the kinetics of hydrolysis. The present research focused on the identification of an esterase from a bacterium, and the identification of the main factors influencing the associated rates of hydrolysis.