Selective inhibition of heme oxygenase, without inhibition of nitric oxide synthase or soluble guanylyl cyclase, by metalloporphyrins at low concentrations.

Studies on the physiological role of heme oxygenase (HO) require an inhibitor that will selectively inhibit HO activity without inhibiting the activity of either nitric oxide synthase (NOS) or soluble guanylyl cyclase (sGC). The objective of this study was to test a series of metalloporphyrins that have previously been shown to inhibit HO activity, for their ability to inhibit HO without inhibiting NOS or sGC activities. Measurement of activity of HO in rat brain microsomes and NOS in rat brain cytosol was made for samples incubated with metalloporphyrins (0.15-50 microM), including zinc protoporphyrin IX, zinc deuteroporphyrin IX 2,4-bis-ethylene glycol (ZnBG), chromium mesoporphyrin IX (CrMP), tin protoporphyrin IX, and zinc N-methylprotoporphyrin IX. CrMP and ZnBG were found to be the most selective inhibitors of HO activity (i.e., caused the greatest inhibition of HO activity, 89 and 80%, respectively, without inhibition of NOS activity). Based on these results, sGC activity in rat lung cytosol incubated with CrMP or ZnBG (0.15-15 microM) was measured. ZnBG did not affect basal sGC activity but did potentiate S-nitroso-N-acetylpenicillamine (SNAP)-induced sGC activity. CrMP did not affect either basal or SNAP-induced activity. It was concluded that of the five metalloporphyrins studied, CrMP, at a concentration of 5 microM, was a selective inhibitor of HO activity and was the most useful metalloporphyrin for the conditions tested. Thus, CrMP would appear to be a valuable chemical probe in elucidating the physiological role of HO.