The catabolism of heme is carried out by members of the heme oxygenase (HO) family. The products of heme catabolism by HO-1 are ferrous iron, biliverdin (subsequently converted to bilirubin), and carbon monoxide. In addition to its function in the recycling of hemoglobin iron, this microsomal enzyme has been shown to protect cells in various stress models. Implicit in the reports of HO-1 cytoprotection to date are its effects on the cellular handling of heme/iron. However, the limited amount of uncommitted heme in non-erythroid cells brings to question the source of substrate for this enzyme in non-hemolytic circumstances. In the present study, HO-1 was induced by either sodium arsenite (reactive oxygen species producer) or hemin or overexpressed in the murine macrophage-like cell line, RAW 264.7. Both of the inducers elicited an increase in active HO-1; however, only hemin exposure caused an increase in the synthesis rate of the iron storage protein, ferritin. This effect of hemin was the direct result of the liberation of iron from heme by HO. Cells stably overexpressing HO-1, although protected from oxidative stress, did not display elevated basal ferritin synthesis. However, these cells did exhibit an increase in ferritin synthesis, compared with untransfected controls, in response to hemin treatment, suggesting that heme levels, and not HO-1, limit cellular heme catabolism. Our results suggest that the protection of cells from oxidative insult afforded by HO-1 is not due to the catabolism of significant amounts of cellular heme as thought previously.In the average adult, at equilibrium ϳ2 million red blood cells are being turned over every second. In a day, this process requires about 25 mg of iron (Fe) to be recycled from the hemoglobin of effete erythrocytes. Heme oxygenase 1 (HO-1) 2 is the enzyme responsible for catabolizing the heme from senescent red blood cells to release Fe, as well as equimolar amounts of carbon monoxide (CO) and biliverdin, for its eventual reuse in erythropoiesis. Another noninducible isoform of heme oxygenase with heme catabolic properties similar to HO-1, HO-2 is present in substantial levels primarily in the central nervous system and testes (1, 2). Although the heme-degrading function of HO has been known since the 1960s (3, 4), only recently has it been shown that heme oxygenases may be involved in cytoprotection against cellular stresses (for review see Camara and Soares (5), Abraham and Kappas (6), Otterbein et al. (7), and Ryter et al. (8)). This newer discovery of the potential of these enzymes as a tissue defense mechanism has spawned a flurry of studies by numerous groups who have shown by several different approaches that induction or overexpression of heme oxygenase 1 can protect tissues from various insults, including oxidative stress and immune system attack.The precise mechanism by which HO-1 mediates its protective function remains controversial. Breakdown of heme being the only known reaction catalyzed by the enzyme, extensive research has alleged that one or mo...