title: Metabolism of the TRH analog L-pyro-2-aminoadipyl-L-histidyl-[3H]-L-thiazolidine-4-carboxamide (MK-771) in gut and brain tissue of rats: the implications for its bioavailability.
abstract: Hydrolysis of the terminal amide group of L-pyro-2-aminoadipyl-L-histidyl-[3H]-L-thiazolidine-4-carboxamide ([3H]MK-771) in rat brain homogenates was rapid and yielded the corresponding [3H] tripeptide carboxylic acid (III). Brain proteolytic enzymes may limit the bioavailability of [3H]MK-771. In contrast MK-771 degradation in a rat gut homogenate (where the radiolabeled product of hydrolysis was [3H]thioproline) was much slower and intestinal proteolytic enzymes probably did not prevent the absorption of MK-771 into the systemic circulation. However, the majority of an oral dose of MK-771 was not absorbed and intact MK-771 represented only 2% of the fecal radioactivity. Degradation of unabsorbed MK-771 occurred mainly in the large intestine of normal rats presumably because of the action of gut flora. Eighty percent of the oral dose remained in the intestine of germ-free rats as intact MK-771 and it was concluded that the limited absorption of MK-771 was caused by its inefficient transportation across gut membranes.
mesh_terms: D000818:Animals; D001682:Biological Availability; D001711:Biotransformation; D001921:Brain / Q000378:metabolism*; D004064:Digestive System / Q000378:metabolism*; D005243:Feces / Q000032:analysis; D008401:Gas Chromatography-Mass Spectrometry; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D053778:Thiazolidines; D013973:Thyrotropin-Releasing Hormone / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D053778:Thiazolidines; D013973:Thyrotropin-Releasing Hormone; C021597:pyro(l-alpha-aminoadipyl)-L-histidyl-L-thiazolidine-4-carboxamide