Effects of the chrysotherapeutic agents auranofin and gold sodium thiomalate on hepatic and renal drug metabolism and heme metabolism

Chemistry and biology of heme. Effect of metal salts, organometals, and metalloporphyrins on heme synthesis and catabolism, with special reference to clinical implications and interactions with cytochrome P-450

Although free porphyrins occur in nature in small quantities, no known function has been assigned to them. In contrast, heme and cobalamin, which are Fe and Co chelates of porphyrins or porphyrin derivatives, respectively, carry out crucial biological functions. Heme is the prosthetic group for a number of hemoproteins. These include myoglobin and hemoglobin, which carry out oxygen binding or transport; mitochondrial cytochromes aa3, b, c, and c3, which are important in transferring electrons; microsomal cytochrome P-450, which catalyzes mixed-function oxidations; catalase, which decomposes H2O2; peroxidase, which activates H2O2; and tryptophan pyrrolase, which catalyzes the oxidation of tryptophan. Recently, heme has also been shown to be the prosthetic group of prostaglandin and peroxide synthetase and indoleamine dioxygenase. The elegant studies of the biochemical pathway for the formation of heme demonstrated the arrangement in the porphyrin macrocycle of the carbon and nitrogen atoms originating from the eight glycine and the succinic acid molecule that are the precursors of porphyrins. There are eight enzymes involved in the synthesis of heme. The first and last three of these enzymes are localized in mitochondria, while the intermediate enzymes are localized in cytosol. The catalytic site of HMOX recognizes metalloporphyrins with central metal atoms other than iron; it favors some of these metalloporphyrins over heme as a potential substrate, sometimes by a large factor, permitting the synthetic heme analogue to serve as a potent competitive inhibitor of HMOX reaction. Since these synthetic metalloporphyrins do not bind molecular oxygen, they are not metabolically degraded by ring rupture and do not add to the body pool of bile pigment. One possible consequence of this competitive inhibition of heme degradation is suppression of bile pigment formation to such a degree that excessive plasma levels of bilirubin may be diminished. The studies of Drummond and Kappas (1981) and later studies in rats, mice, monkeys, and man, and also our studies have proved the latter phenomenon. The compound does not appear to affect the metabolic disposition of preformed bilirubin but inhibits biliary bilirubin excretion derived from the metabolism of endogenous or exogenous heme. Whether some of the effect of Sn-PP on naturally occurring or experimentally induced jaundice in animals reflects diversion of heme to nonheme to oxygenase-dependent pathways of heme metabolism, or whether a pathway which is normally latent becomes activated concurrent with HMOX inhibition is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of heme oxygenase gene expression by cobalt in rat liver and kidney

The effect of heavy metals such as cobalt chloride and the corresponding metalloporphyrin on the transcription of the heme oxygenase gene in tissues was examined using cDNA for rat heme oxygenase as the probe. An increase in heme oxygenase mRNA level was observed in response to cobalt chloride and cobalt protoporphyrin treatment in both liver and kidney. Quantitative evaluation of the heme oxygenase transcript was obtained by determining the intensity of mRNA bands by scanning densitometry, and indicated that cobalt chloride increased heme oxygenase mRNA by 40-60-fold after 2 h of metal exposure. Accumulation of heme oxygenase mRNA after cobalt chloride administration was prevented by co-administration of actinomycin D or cycloheximide. These results indicate that the increased expression of heme oxygenase by cobalt chloride required de novo protein synthesis and was regulated at the transcriptional level. The time course of heme oxygenase transcript accumulation following administration of cobalt protoporphyrin was different from that of cobalt chloride. There was a sharp increase in heme oxygenase mRNA after cobalt chloride administration at 2 h and cobalt protoporphyrin at 10 h. Heme, to which cobalt protoporphyrin is structurally analogous, acted as a potent inducer of heme oxygenase transcripts in both liver and kidney. Variation in heme oxygenase mRNA levels resulting from enhanced transcription of the heme oxygenase gene was evaluated by nuclear runoff assay using isolated rat liver nuclei after cobalt chloride administration. Quantification of specific nuclear RNAs labeled during the in vitro transcription revealed active heme oxygenase gene transcription in liver nuclei from cobalt-chloride-treated rats. Transcription of heme oxygenase is greatly increased within 1 h of administration of cobalt chloride in rat liver, as evidenced by the level of [alpha-32P]UTP incorporation into nuclear RNA. The transcription was increased by 40-fold after 3 h of cobalt chloride administration. The activation of the heme oxygenase gene by metal ions is the most rapid transcriptional response to heavy metals yet described and highlights the regulatory role of heme oxygenase in heme degradation during deviating environmental conditions. On the other hand, cobalt protoporphyrin and heme arginate increase transcription of the heme oxygenase gene in a similar pattern but at a slower rate than that of the heavy metal, suggesting that the heme oxygenase promotor region may contain additional elements conferring the inducing effect of these two agents.

Properties of human kidney heme oxygenase: inhibition by synthetic heme analogues and metalloporphyrins

Heme oxygenase activities in human kidney microsomes were found to be from 0.238 to 0.620 nmol of bilirubin/mg/hr (mean 0.375, SD 0.134), which represent approximately 30% of activities determined for human adult liver. There was interindividual variation in heme oxygenase activity of a 2-5-fold difference. Rabbits were immunized with purified human liver heme oxygenase and the resulting antibody preparation was used to examine the species specificity of the enzyme. Microsomal protein with a molecular weight of 32,000 from human kidney was identified on Western blots by its reaction with the anti-heme oxygenase liver antibody similar to the purified enzyme protein. Thus, a homology exists between human hepatic and kidney heme oxygenase. The enzyme activity was sensitive to inhibition by metalloporphyrins, such as tin-protoporphyrin IX and, to a lesser degree, by zinc and cobalt protoporphyrin IX. In a study of different synthetic heme analogues for in vitro inhibition of heme oxygenase, we found that replacement of iron by zinc in deuteroporphyrin IX 2,4 bis glycol dramatically potentiated the inhibition of heme oxygenase activity. This finding demonstrated that zinc deuteroporphyrin IX 2,4 bis glycol is a most potent inhibitor of heme oxygenase activity.

Metal induction of heme oxygenase

The experimental data that have been reviewed support the following conclusions regarding metal induction of microsomal heme oxygenase activity: 1. Induction of heme oxygenase in liver, kidney, and other organs of rodents is a nonspecific, toxic response to parenteral administration of numerous metal compounds. 2. The Co2+ and Cd2+ ions are especially potent for induction of heme oxygenase in rat liver; Sn2+, Ni2+, and As3+ are especially potent for induction of the enzyme in rat kidney; Hg2+ is especially potent for induction of the enzyme in rat adrenal. 3. Rat spleen, testis, and brain are relatively refractory to metal induction of heme oxygenase activity; in testicular microsomes from Cd2+-treated rats, heme oxygenase activity is markedly inhibited. 4. Metal induction of heme oxygenase requires de novo synthesis of mRNA and protein, based on 1) experiments with metabolic inhibitors (actinomycin D, puromycin, and cycloheximide) and 2) translation assays of heme oxygenase mRNA. 5. Heme oxygenase induction by metals is generally suppressed by treatments with SH compounds (for example, cysteine and glutathione) and enhanced by agents that deplete tissue SH levels (for example, diethyl maleate), suggesting that the induction mechanism may involve binding of metal ions to SH-containing regulatory molecules. 6. Administration of DDC exerts a pronounced synergistic effect on Ni2+ induction of heme oxygenase activity in rat tissues, attributable in part to enhanced cellular uptake of nickel. 7. Induction of heme oxygenase is not sustained during repeated daily treatments of rats with NiCl2, pointing to an adaptive mechanism for tolerance to the enzyme inducer. Investigations of metal induction of heme oxygenase activity have commonly involved sc or ip administration of the test compounds to rats. The paucity of studies in other species and the dearth of experiments with oral or respiratory exposures hinder extrapolations of the animal data to human environmental or occupational exposures to metal compounds.