Heme arginate and the endothelium: mechanism for its safety in porphyria

Acute intermittent porphyria (AIP) is a potential fatal disease characterized by decreased synthesis of heme and accumulation of porphyrin precursors. Infusion of hematin has proven efficacious, but with considerable vascular side effects such as thrombosis and DIG; in contrast, Finnish investigators have demonstrated that heme arginate (HA) is both effective and non-vasculotoxic. We have previously shown that heme (hemin chloride) serves as a catalytically active iron source, potentiating the oxidation of LDL and sensitizing endotnelial cells (EC) to oxidant injury. EC respond to heme and oxidant challenge by upregulating the cell cytoprotectants, heme oxygenase (HO) and ferritin. We now provide a mechanism that accounts for the relative safety and efficacy exhibited by HA treatment but not heme in AIP. HA does not amplify EC cytotoxicity mediated by H₂₂O₂ while heme pretreatment is markedly sensitizing (5.3+2.3 vs. 52.3±5.3% 5'Cr release respectively). Nevertheless, HA enters EC similarly to heme, since both markedly induce HO mRNA (more than 20-fold increase) and enzyme activity (more than 5-fold increase). Hydrophilic heme analogues such as iron-protoporphyrin IX-bis-sulfonate and -bis-glycol do not amplify EC cytolysis and do not enhance HO gene expression. Despite efficient entry in EC, HA marginally increases EC ferritin content (74.9±9 vs. 41+11 ng/mg cell protein compared to heme (1517+20 ng/mg cell protein) suggesting a lower level of iron release from the porphyrin ring. HA is only 1β as catalytically active as heme in oxidizing LDL in the presence of H₂p2. Ferritin insignificantly increases in EC exposed to HA/H₂₂O₂-conditioned LDL while, LDL conditioned with heme/H₂₂O₂ significantly induces EC ferritin synthesis (1,9 fold). HA/H₂₂O₂-conditioned LDL is significantly (p<.01) less cytotoxic (21.9±6.8%) than heme/H₂₂O₂-conditioned LDL (57.6±4,1%). We conclude that HA is not as potent a free radical catalyst as heme. We speculate that HA may not potentiate EC damage or oxidation of lipids in vivo, making it a preferred choice for treatment of AIP compared to heme.