title: Effect of cimetidine on delta-aminolevulinic acid synthase and microsomal heme oxygenase in rat liver.
abstract: Cimetidine is a well known inhibitor of the heme-containing enzyme cytochrome P-450. We have found that it also inhibits delta-aminolevulinic acid synthase (ALA-S) and microsomal heme oxygenase, the rate-limiting enzymes for heme synthesis and heme degradation respectively. Cytochrome P-450 content was decreased but microsomal heme concentration remained unaltered for a period of 30 min after in vivo cimetidine administration to rats. In vitro incubation of cimetidine with each of the above enzymes revealed no direct effect of cimetidine on ALA-S but about 50% inhibition of heme oxygenase and 20% reduction in cytochrome P-450 content. This suggests that a metabolite of cimetidine inhibits ALA-S activity in vivo, while the drug itself or a metabolite inhibits heme oxygenase both in vivo and in vitro. A rise in ALA-S activity seen after its early inhibition and its return to approximate control values after 60 min suggest a reversible inhibition of ALA-S by a metabolite of cimetidine and may correspond to its clearance from the animal. An elevation in microsomal heme content paralleled the rise in ALA-S activity while microsomal heme oxygenase activity returned to only 65% of control value 60 min after cimetidine treatment. Cytochrome P-450 content did not change after its initial decrease, suggesting that irreversible alteration had occurred.
mesh_terms: D000624:5-Aminolevulinate Synthetase / Q000037:antagonists & inhibitors*; D000818:Animals; D002927:Cimetidine / Q000494:pharmacology*; D065607:Cytochrome P-450 Enzyme Inhibitors; D004305:Dose-Response Relationship, Drug; D006419:Heme Oxygenase (Decyclizing) / Q000037:antagonists & inhibitors*; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors*; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D002927:Cimetidine; D006899:Mixed Function Oxygenases; D006419:Heme Oxygenase (Decyclizing); D000624:5-Aminolevulinate Synthetase