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Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)

Effect of cimetidine on delta-aminolevulinic acid synthase and microsomal heme oxygenase in rat liver.

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.[1]


  1. Effect of cimetidine on delta-aminolevulinic acid synthase and microsomal heme oxygenase in rat liver. Marcus, D.L., Halbrecht, J.L., Bourque, A.L., Lew, G., Nadel, H., Freedman, M.L. Biochem. Pharmacol. (1984) [Pubmed]
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