The Enzymatic Degradation of Hemoglobin to Bile Pigments by Macrophages

Abstract

Recent studies have identified and characterized the enzymatic mechanism by which hemoglobin-heme is converted to bilirubin. Under physiologic conditions the enzyme system, microsomal heme-oxygenase, is most active in the spleen followed by the liver and bone marrow, all of which are tissues that normally are involved in the sequestration and metabolism of red cells. Indirect evidence suggested that the reticuloendothelial system is important in this process. To test this hypothesis, conversion of heme to bilirubin was studied in macrophages obtained by chemical or immunological means from the peritoneal cavity or from the lungs of rodents. Homogenates of pure populations of these cells were devoid of heme-oxygenase activity, unless before harvesting the macrophages had been exposed to methemalbumin, microcrystalline hemin, or hemoglobin in vivo. In macrophages exposed to heme pigments, the specific activity of heme-oxygenase was far in excess of that in the spleen or liver. Enzyme activity was also present in the granulomatous tissue surrounding subcutaneous hematomas. The heme-oxygenase system in macrophages resembles that in the spleen and liver in that it is localized in the microsomal fraction, has an absolute requirement for molecular oxygen and NADPH, is inhibited by carbon monoxide, and has a similar Km. These findings indicate that cells of the reticuloendothelial system, presumably including the Kupffer cells of the liver and the macrophages of the spleen, possess the enzymatic machinery for converting hemoglobin-heme to bilirubin. The reaction is a mixed function oxidation, probably involving cytochrome P450 as the terminal oxidase. Enzyme activity in macrophages is capable of regulatory adaptation in response to substrate loads. In the standard assay system for the enzyme, disappearance of heme always was in excess of the amount of bilirubin formed, suggesting the simultaneous presence of alternate routes of heme degradation not involving bilirubin as an end product or intermediate.

In Vitro Synthesis and Secretion of Lysozyme by Mononuclear Phagocytes

“…No specific inducer molecules could be identified in these experiments since lysozyme production remained relatively high in basal media and could only be blocked by inhibiting the synthesis of all proteins. The apparent stability of lysozyme production and secretion can be contrasted with the inducibility of other macrophage enzymes like acid phosphatase (22) and heme oxygenase (23). Several observations make it likely that lysozyme secretion is independent of endocytosis and the production of acid hydrolases.…”

Erythrocyte Catabolism by Macrophages In Vitro THE EFFECT OF HYDROCORTISONE ON ERYTHROPHAGOCYTOSIS AND ON THE INDUCTION OF HEME OXYGENASE

“…The third explanation postulating operation of alternate pathways for heme degradation appears the most plausible, since the existence of such degradative pathways which do not lead to formation of bilirubin has already been demonstrated (46,47). Although the sequence of the chemical reactions and the nature of the intermediates involved are as yet unknown, it has previously been reported that on incubation of heme with homogenates of stimulated macrophages, heme disappearance substantially exceeds the rate of bilirubin formation (15).…”

“…The mechanism of this conversion was obscure until the recent demonstration by Pimstone, Tenhunen, Seitz, Marver, and Schmid (15), that macrophages possess HO, which is the enzyme system catalyzing the conversion of ferroprotoporphyrin (heme) to bilirubin IXa (13,14). Although HO activity is very low in native peritoneal macrophages, presumably because they normally lack contact with heme compounds, a striking stimulation of enzyme activity was observed after macrophages had been exposed to methemalbumin or hemoglobin in the peritoneal cavity (15).…”

“…Events involved in the into the surrounding extracellular space (11), and iron, presumably derived from the degraded hemoglobin, accumulates within the phagocytes (12). In addition, heme oxygenase (HO),' the enzyme system that converts heme to bilirubin IXa (13,14), is stimulated by hemoproteins ingested by the macrophages ( 15).…”

Az oxidatív stressz és az antioxidáns védelmi rendszer vizsgálata nehézfém kezelést követően pontyban és streptozotocin-indukálta diabéteszes patkány modellben

“…Általánosságban elmondható, hogy alap expressziója lépben és a májban nagyobb, mint a többi szervben [42,164]. A HO-1 egy hősokk fehérje (HSP 32), mely számtalan stimulus hatására indukálódik, úgymint nehézfémterhelés, hősokk, ischémia, GSH szint csökkenés, sugárzás, hipoxia, hiperoxia és egyéb patológiás állapotokat okozó inzultusok [120].…”