Metabolism of dibasic esters by rat nasal mucosal carboxylesterase

Inhalation exposure of rats to dibasic esters revealed lesions of the nasal olfactory epithelium similar to those observed with other ester solvents. Female rats are more sensitive to these effects than are male rats. It has been proposed that carboxylesterase conversion of inhaled esters within nasal tissues to organic acids may be a critical biochemical step in converting these chemicals to toxic substances. These experiments measured the kinetic parameters Vmax, KM, Ksi, and V/K for the hydrolysis of the dibasic esters in the target nasal tissue, olfactory mucosa, and nontarget tissue, respiratory mucosa. It was determined that under the conditions of these experiments, diacid metabolites are not formed. Esterase activity was inhibited by pretreatment with bis p-nitrophenyl phosphate. Vmax values for the three dibasic esters were 5- to 13-fold greater in olfactory mucosa than respiratory mucosa for male or female rats. V/K values were 4- to 11-fold greater in olfactory mucosa than respiratory mucosa for male or female rats. V/K was similar between male and female olfactory mucosa when dimethyl glutarate was used as the substrate. With dimethyl succinate or dimethyl adipate as the substrate, V/K for female olfactory tissue was 0.5- or 2-fold that of males, respectively. Differences in V/K were mainly due to decreases in KM associated with increasing carbon chain length. Substrate inhibition was observed at dibasic ester concentrations greater than approximately 25 mM, which are unlikely to be achieved in vivo. These results lend further support to the hypothesis that organic acid accumulation in the target tissue, olfactory mucosa, plays a significant role in the pathogenesis of dibasic ester-induced nasal lesions. This mechanism may be applicable to a wide range of inhaled esters.