Effect of cobalt protoporphyrin on hepatic drug-metabolizing enzymes. Specificity for cytochrome P-450.

Cobaltic protoporphyrin IX (cobalt protoporphyrin) is known to cause an extensive and long-lasting depletion of hepatic cytochrome P-450 in rats, and it has been used to evaluate the role of hepatic cytochrome P-450 in xenobiotic metabolism and toxicity. To examine the specificity of cobalt protoporphyrin for hepatic cytochrome P-450, cobalt protoporphyrin was administered to rats and hamsters, and its effects on cytochrome P-450-dependent and non-P-450-dependent phase I and phase II metabolism were determined. Cobalt protoporphyrin pretreatment depleted hepatic cytochrome P-450 in both species and lowered their Vmax values for the hepatic microsomal metabolism of ethylmorphine, aminopyrine, ethoxyresorufin and ethoxycoumarin, without change in their Km values. In the rat, cobalt protoporphyrin treatment lowered both the Vmax and Km for microsomal metabolism of aniline. In vivo hepatic cytochrome P-450-dependent metabolism, as measured by antipyrine clearance, was decreased in both species. UDP-Glucuronyltransferase, phenolsulfotransferase and glutathione-S-transferase were unaffected, as was hepatic glutathione. Modest effects of cobalt protoporphyrin were seen on the hepatic microsomal flavoprotein mixed-function oxidase (hamster only), cytochrome P-450 reductase, cytochrome b5 (rat only), UDPGA (rat only), and glycogen, and on blood glucose (rat). In in vivo studies with hamsters given a low dose of acetaminophen, cobalt protoporphyrin suppressed the apparent rate constants for the cytochrome P-450-dependent pathways of acetaminophen metabolism but had no effect on acetaminophen glucuronidation and sulfation. Polyacrylamide gel electrophoresis analysis indicated that cobalt protoporphyrin markedly reduced the levels of the cytochrome P-450 holoenzyme but did not alter either the content or profile of the cytochrome P-450 apoenzyme. collectively, the data indicate that cobalt protoporphyrin shows relatively high selectivity for the hepatic cytochrome P-450 system, and support the use of this compound as a tool for resolution of the role of hepatic cytochrome P-450 in xenobiotic metabolism and toxicity.