Plant biochemistry of xenobiotics. Purification and properties of a wheat esterase hydrolyzing the plasticizer chemical, bis(2-ethylhexyl)phthalate

Abstract

A soluble wheat esterase, catalyzing a cleavage of the mass-produced plasticizer chemical, bis(2-ethylhexy1)phthalate (DEHP), has been discovered. Although wheat plants and seeds as well as cultured wheat cells contained more than 12 non-specific esterase activities, only a single protein with a marked preference for a substrate chain-length of 6-8 carbon atoms was active with DEHP. This enzyme is shown to differ from all previously characterized plant lipases and esterases.The enzyme was purified 10-fold from wheat plants and 280-fold, to electrophoretic homogeneity, from cultured wheat cells. An apparent functional molecular mass of 38000Da and an apparent subunit molecular mass of 22000 D a were determined. Inhibitor experiments pointed to the catalytic involvement of a serine residue. Cleavage of DEHP by the purified enzyme was about lo4 times slower than cleavage of 4-nitrophenyl octanoate. This was consistent with previous evidence for a rate-limiting role of the esterase reaction in DEHP metabolism by intact wheat cellsThe fate of environmental chemicals in plants is of great practical concern in view of the huge global plant biomass and of the widespread use of pesticides in agriculture. It is clear that plants play an important role in the transformation of environmental chemicals [I, 21. However, the state of biochemical knowledge on the plant metabolism of xenobiotics is poor when compared to the detailed knowledge on the mammalian metabolism of xenobiotics [3]. Only few plant enzymes acting on xenobiotics have so far been purified and characterizedThe present report deals with a plant enzyme acting on the mass-produced plasticizer chemical, bis(2-ethy1hexyl)phthalate (DEHP, Fig.l). The 1979 production figure of DEHP was about 300 million pounds for the US alone [7]. DEHP is found world-wide as a contaminant of sediments and air. Phthalates, including DEHP, have also repeatedly been isolated from plant material (e.g. [8]), but many of these reports may have been due to contamination of the solvents used for extraction. The occurrence and toxicology of phthalate diesters have been thoroughly reviewed [7, 9, 101. The plant metabolism of phthalate diester plasticizers has only very recently been studied. A small amount (1.2%) of metabolites was present in the plant component (Elocku canademis) of a model ecosystem [l I]. Much higher amounts of DEHP conversion products were isolated from the plant components (Chara chara, Mentha aguatica) of another model ecosystem [12]. However, it remained unclear whether these products had been formed by the plant, by other components of the model ecosystems or through abiotic mechanisms. DEHP [4-61.Abbreviations. DEHP, bis(2-ethylhexy1)phthalate; MEHP, mono-(2-ethylhexy1)phthalate.

Catalytic characteristics of plant-esterase from wheat flour

“…More than 70% activity was determined in the temperature range 20-50°C, while only 22 and 15% activity were assayed at 10 and 70°C, respectively. Similar temperature profiles have been reported for an esterase extracted from tomato (Preiss et al 1988) and an esterase from wheat cells (Krell and Sandermann 1984). These enzymes show immense similarities on the optimal temperature among species and under the same species, which may be due to their similar functions in catalytic reaction.…”

Role of a Carboxylesterase in Herbicide Bioactivation in Arabidopsis thaliana

“…An important group of phase 1 enzymes that have received very little attention in plants are the xenobiotic-hydrolyzing carboxylesterases (CXEs). 3 These plant enzymes detoxify persistent pollutants (3) and insecticides (4), as well as hydrolyzing pro-herbicide esters to their bioactive free acids (5,6). In the latter case, many major classes of herbicides are applied as esters to facilitate penetration into the leaf.…”

A Mono-2-Ethylhexyl Phthalate Hydrolase from a Gordonia sp. That Is Able To Dissimilate Di-2-Ethylhexyl Phthalate

“…The purified specific esterases from mouse hepatic microsomes and human saliva have activity that hydrolyzes phthalate diesters to the corresponding monoesters (18,29). Purified wheat plant esterase exhibits activity that hydrolyzes DEHP to mono-2-ethylhexyl phthalate (MEHP) (20). Higher organisms, such as mammals and plants, metabolize phthalate diesters to the corresponding monoesters.…”