The extent and implications of interspecies differences in the intestinal hydrolysis of several glucuronide conjugates

On the basis of in vitro data, large interspecies differences have been projected in $\beta$-glucuronidase activity in the intestine of man and rabbits relative to rats and mice. High $\beta$-glucuronidase activity has the potential to result in amplification of the toxicity of compounds excreted into the bile as glucuronide conjugates since the hydrolyzed aglycone would be available for reabsorption and enterohepatic circulation. This research examined the capacity of the microbiota from the proximal and distal small intestine and cecum of mice, rats and rabbits to hydrolyze three glucuronide conjugates under culture conditions closely resembling the in vivo gut environment. Hydrolysis of glucuronide conjugates in the proximal and distal small intestine was uniformly low in all species relative to the cecum. There were species and substrate specific differences in the hydrolysis of glucuronide conjugates in the cecum which may have important implications for animal extrapolation. As an adjunct to the above research, studies were conducted comparing the variance of $\beta$-glucuronidase activity for different methods of calculation of bacterial enzyme activity. Under aerobic assay conditions, calculation of activity per gram of contents was associated with significantly lower variance compared to activity calculated per mg of protein as measured by the coefficient of variation. However, under anaerobic incubation conditions none of the methods of calculating enzyme activity (per gram of contents, per mg of protein, per 10$\sp{10}$ cells) was associated with significantly lower variance relative to the others. Studies were also conducted to compare the influence of aerobic versus anaerobic incubation conditions on intestinal $\beta$-glucuronidase activity. In general, it was found that aerobic incubation conditions are associated with significantly higher intestinal $\beta$-glucuronidase activity, possibly due to the liberation of the enzyme during cell lysis which occurs more readily under aerobic incubation conditions