The effects of selected heme analogues on heme oxygenase activity in tissues and on human and rabbit bone marrow hematopoietic colony growth were examined. Zinc protoporphyrin (ZnPP) and zinc mesoporphyrin (ZnMP), at concentrations ranging between 1 and 20 M, produced significant inhibition of human and rabbit bone marrow erythroid (CFU-E, BFU-E) and myeloid (CFU-GM) colony growth. The growth inhibition produced by ZnPP or ZnMP was not overcome with exposure of cultures to elevated levels of the growth factors erythropoietin and granulocytemacrophage colony stimulating factor. In contrast, tin protoporphyrin and tin mesoporphyrin did not display any significant bone marrow toxicity when used at similar concentrations. In other studies, differential effects of tin mesoporphyrin and ZnMP administered intravenously on kidney heme oxygenase were demonstrated. Chromium mesoporphyrin administered intravenously proved lethal to animals. These results indicate that exposure of bone marrow to ZnPP and ZnMP can be deleterious to hematopoietic cells and raise the possibility that ZnPP, which is endogenously formed and found in high concentration in red blood cells in lead-poisoned children, may itself participate in the bone marrow toxicity produced by this metal.Hematopoietic cell growth and differentiation within the bone marrow microenvironment are dependent on a complex interplay of cells, cytokines, growth factors, and heme oxygenase (HO) activity, with the latter enzyme playing a major regulatory role in this system. Heme, a potent inducer of HO expression has been shown to have modulatory effects on hematopoiesis (1). A comprehensive study comparing the effects on hematopoietic cells of synthetic heme analogues, which inhibit HO activity has, not previously been undertaken. Information from this type of study is of special importance because of the clinical potential (2-8) of certain of these compounds.In this study we compared the effects of tin and zinc porphyrins on hematopoietic cell growth and colony formation in animal and human bone marrow cultures. Such cell systems are especially vulnerable to the nature of their microenvironment and thus can provide sensitive indices of the deleterious potential of various chemical agents.The results of this study indicate that zinc porphyrins are toxic to both myeloid and erythroid cell growth even at low concentrations. In contrast, tin porphyrins, even at high concentrations, displayed no toxic effects on hematopoiesis. In other experiments tin and zinc porphyrins were shown to have differing effects on renal HO activity when administered intravenously (i.v.). Chromium mesoporphyrin (CrMP) proved lethal to animals when administered by the iv route.These findings provide additional examples of the differential effects of HO inhibitors on cell functions based on their central metal atom and on their route of administration. The inhibitory actions of zinc porphyrins on bone marrow cell growth represent newly identified deleterious properties of these metalloporphyr...