Analysis of vinyl acetate metabolism in rat and human nasal tissues by an in vitro gas uptake technique

Physiologically based pharmacokinetic (PBPK) models require estimates of ca talytic rate constants controlling the metabolism of xenobiotics. Usually, these constants are derived from whole tissue homogenates wherein cellular architecture and enzyme compartmentation are destroyed. Since the nasal cav ity epithelium is composed of a heterogeneous cell population measurement o f xenobiotic metabolizing enzymes using homogenates could yield artifactual results. In this article a method for measuring rates of metabolism of vin yl acetate, a metabolism-dependent carcinogen, is presented that uses whole -tissue samples and PBPK modeling techniques to estimate metabolic kinetic parameters in tissue compartments. The kinetic parameter estimates were com pared to those derived from homogenate experiments using two methods of tis sue normalization. When the in vitro gas uptake constants were compared to homogenate-derived values, using a normalization procedure that does not ac count for tissue architecture, there was poor agreement. Homogenate-derived values from rat nasal tissue were 3- to 23-fold higher than those derived using the in vitro gas uptake method. When the normalization procedure for the rat homogenate-derived values took into account tissue architecture, a good agreement was observed. Carboxylesterase activity in homogenates of hu man nasal tissues was undetectable. Using the in vitro gas uptake technique , however, carboxylesterase activity was detected. Rat respiratory carboxyl esterase and aldehyde dehydrogenase activities were about three and two tim es higher than those of humans, respectively. Activities of the rat olfacto ry enzymes were about equivalent to those of humans. K-m values did not dif fer between species. The results suggest that the in vitro gas uptake techn ique is useful for deriving enzyme kinetic constants where effects of tissu e architecture are preserved. Furthermore, the results suggest that caution should be exercised when scaling homogenate-derived values to whole-organ estimates, especially in organs of cellular heterogeneity, (C) 1998 Society of Toxicology.