DEHP esterase

Because the metabolism of DEHP was catalyzed by so many fractions of the trout liver homogenate, these fractions were characterized by measurement of marker enzymes to determine which organelles actually were responsible for the observed DEHP metabolism. Succinic dehydrogenase activity was used as a marker for mitochondria, whereas glucose-6-phosphatase was used as a marker for microsomes. The distribution of DEHP oxidase activity (production of polar metabolites 1 and 2 with added NADPH) and of DEHP esterase activity (production of monoester without added NADPH) were also determined. It was found (Figure 2) that the distribution of DEHP oxidase activity parallels the distribution of microsomal activity and the distribution of DEHP esterase activity parallels the distribution of microsomal activity, but is also present in the cytosol fraction. 

Figure 2. Distribution of marker enzymes and DEHP-metabolizing enzymes in trout liver homogenate fractions. DEHP esterase and DEHP oxidase were each measured by 1-hr incubations of 0.010 ftmol of UC-DEHP in a total volume of 2 mL. Fraction (A), 2,000 g pellet (B), 10,000 g pellet (C), 100,000 g pellet and (D), 100,000 g supernatant. Relative Specific Activity = percent of total activity/ percent of total protein (14).

Bis(2-ethylhexyl)-phthalate esterase DEHP esterase Acts also on long-chain 4-nitrophenyl esters... 

It was also found that paraoxon, an esterase inhibitor, substantially reduced formation of polar metabolite 1 from DEHP by trout liver microsomes with added NADPH. This suggests that polar metabolite 1 is formed via further metabolism of the monoester, the production of which was reduced by paraoxon. 

Fish liver microsomes are capable of both hydrolytic and oxidative metabolism of phthalate esters. In addition, trout liver cytosol and blood serum exhibited esterase activity against DEHP. 

Eor example, in the intestinal tract and liver of both humans and animals DEHP is rapidly hydrolyzed by esterases to yield mono-(2-ethylhexyl) phthalate (MEHP) and 2-ethylhexanol [25]. The latter metabolite is subsequently oxidized enzymatically to 2-ethyl hexanoic acid (2-EHXA) [26]. MEHP, 2-hethylhexanol, and/or their metabolites are the immediate inducers of the majority of enzymes known to be affected by exposure of DEHP [27]. Due to the high importance of the primary and secondary PAE metabolites in the human exposure smdies, during the last years a big number of smdies have been conducted to prove that some of them are appropriate biomarkers to calculate human PAE intake [28-30] and that their determination is easier than calculate it through food intake, which are more time consuming and subjects to several error sources. 

No data were located regarding the metabolites produced in humans or animals after either inhalation or dermal exposures to DEHP. Metabolism following these routes of exposure is expected to be similar to that after oral exposures, since there are lipases present in the alveolar cells of the lungs and the epidermis. However, the activities of these lipases are about 20% of that for the pancreatic esterase secreted into the intestines (Albro et al. 1987), so it is possible that a larger portion of an absorbed dose from respiratory or dermal exposures to DEHP will initially be presented to the tissues as unhydrolyzed DEHP rather than MEHP. 

As DEHP metabolism in human blood is accelerated by blood enzymes, such as esterase, its metabolites are frequently measured. In the study where the metabolites of deuterium-substituted DEHP in urine and in blood serum were compared after oral administration of DEHP, MEHP was found to be the main metabolite in blood serum, existing in greater quantities than MEHHP and MEOHP detected in urine. Therefore, we decided to measure DEHP and MEHP in blood serum. Four healthy volunteers were asked to consume food in containers that were made in part of plastic, and DEHP and MEHP concentrations in blood sera were measured. The concentration of DEHP was equal to or less than the quantitative lower limit (trace level) in all the volunteers. The concentration of MEHP was trace in three of the four volunteers and was lower than the quantitative lower limit in one volunteer. In addition, we measured DEHP and MEHP in human blood plasma samples using LC-MC with the column-switching system as pretreatment. DEHP and MEHP concentrations in blood plasma sampled from six healthy volunteers were equal to or less than the quantitative limit (DEHP <25 ng/ml, MEHP <5 ng/ml). " The results prove that DEHP is rapidly metabolized so that the blood concentration is low and quantification is difficult at normal exposure levels. Urine samples are more suitable for the evaluation of the exposure index of PEs than blood samples. 

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