Metabolism of xenobiotics by the small intestine.

A method was developed for preparing microsomes from rat and guinea-pig small intestine. The microsomal preparations were used to compare rat and guinea-pig drug metabolising enzymes. Guinea-pig intestine was found to have a higher level of cytochrome P-450, cytochrome b5and NADPH cytochrome c reductase than rat; ethoxycoumarin O-deethylase and naphthol glucuronyltransferase levels were similar in both species and deesterification was higher in rat than in guinea-pig intestine. A method was developed for separating villous tip and crypt cells from rat intestine, from which microsomes were prepared and used to study the drug metabolising capacity of the two cell populations. Considerable xenobiotic metabolising activity was found in the intestinal crypt cells, but this activity was 50-70% of that of the villous tip cells. A method for isolating viable cells from guinea-pig small intestine was developed, and these cells were used to study the metabolism of some model compounds. Phase I metabolism of 7-ethoxycoumarin (O-deethylation) was found to be forty times slower than phase II metabolism (glucuronidation and sulphation). Sulpho-transferase did not appear to be readily saturated, whereas glucuronidation did become saturated. 2-Hydroxy-biphenyl was not a good candidate for sulphation, whereas other phenols used were well sulphated. The cytosol fraction of guinea-pig intestinal cells was used to further investigate sulphation. Evidence for the existence of more than one intestinal, cytosolic sulphotransferase was found, and substrate inhibition of the enzymes was observed. Isolated in situ loops of rat and guinea-pig small intestine were used to investigate the fate of 7-hydroxycoumarin conjugates, formed in the intestinal epithelia. A considerable amount of both sulphate and glucuronic acid conjugates reemerged into the intestinal lumen, rather than pass into the blood. This confirmed observations made using everted sacs of guinea-pig small intestine