Most of the CO excreted by mammals is a byproduct of the degradation of heme to form bilirubin. The rate-limiting enzyme in this catabolic pathway is heme oxygenase (HO), which can be inhibited by synthetic metalloporphyrin derivatives of heme (Mps). Therefore, Mps are being studied for use in the suppression of hyperbilirubinemia. There are two forms of HO which are present in differing amounts and proportions in various tissues. In the mouse brain, the predominant isozyme is the constitutive HO-2, whereas, in the mouse spleen, it is the inducible HO-1. We studied the effects of 4 Mps [tin mesoporphyrin (SnMP), chromium mesoporphyrin (CrMP), zinc protoporphyrin (ZnPP), and zinc deuteroporphyrin bis glycol (ZnBG)] on these isozymes using the spleen and brain as sources of HO-1 and HO-2, respectively. HO activity was determined by measuring CO produced in the vial headspace by gas chromatography after tissue sonicates were incubated in the dark with varying concentrations of each porphyrin (n ≥ 5). The concentration of each porphyrin needed to inhibit CO production by 50% (I50) was then determined. Statistical significance was calculated using a two-tailed unpaired t-test (p ≤ .05).
All 4 Mps showed greater inhibitory potency (lower I50) towards HO-2 (brain) versus HO-1 (spleen). Both SnMP and ZnPP were found to exhibit the greatest specificity between isozymes (≈2x). The same order of potency was observed in both isozymes with SnMP showing the highest and ZnPP the lowest. CrMP lacked significant differentiation in I50 between isozymes (*) and when compared to ZnBG for HO-1 (†). All other comparisons were significant. Of note, the I50's of CrMP and ZnPP for both isozymes were significantly different from that previously found in the rat. We conclude that there is a wide spectrum of inhibitory potency and a fair level of selectivity among Mps with the constitutive HO-2 being more susceptible to inhibition than HO-1. These findings should be considered when comparing studies of Mps performed in rat and mouse models and in developing selective inhibitors of HO-1 for therapeutic purposes.
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