Isolation and identification of hemin as an endogenous Na+/K(+)-ATPase inhibitor from porcine blood cells

Myocyte injury by hemin

Effects of free hemin on myocardium were investigated using a model of neonatal myocyte primary cultures. Cells were subjected to free hemin at concentrations up to 20 microM and equilibrated for 5 h at 37 degrees C. Distribution of hemin in media, cell sarcolemma, and cell interior was evaluated. Time-dependent reduction in beating rate was monitored throughout the entire concentration range of administrated hemin. With time and in a hemin concentration-dependent manner, arrhythmic beatings which were followed by loss of contractility were observed. In parallel, morphologic changes appeared from granulation to complete loss of cell integrity. At the concentration range studied, hemin also induced a biphasic release of cytosolic enzymes. In the first phase, the fraction of enzyme released was dependent of the ratio of hemin:cells and was correlated with the amount of nonviable cells as monitored by a trypan blue test. In the second phase, the fraction of released enzyme was much larger than that of nonviable cells. The data are interpreted as an indication of complete loss of cytosolic content due to sarcolemma damage in first phase and partial damage to cell interior in the prolonged second phase. It is concluded that in similarity with other amphipathic molecules, free hemin is toxic to the myocardium.

Inhibition by chloroquine of a novel haem polymerase enzyme activity in malaria trophozoites

The incidence of human malaria has increased during the past 20 years; 270 million people are now estimated to be infected with the parasite. An important contribution to this increase has been the appearance of malaria organisms resistant to quinoline-containing antimalarials such as chloroquine and quinine. These drugs accumulate in the acid food vacuoles of the intraerythrocytic-stage malaria parasite, although the mechanism of their specific toxicity in this organelle is uncertain. The primary function of the food vacuole is the proteolysis of ingested red cell haemoglobin to provide the growing parasite with essential amino acids. Haemoglobin breakdown in the food vacuole releases haem, which if soluble can damage biological membranes and inhibit a variety of enzymes. Rather than degrading or excreting the haem, the parasite has evolved a novel pathway for its detoxification by incorporating it into an insoluble crystalline material called haemozoin or malaria pigment. These crystals form in the food vacuole of the parasite concomitant with haemoglobin degradation, where they remain until the infected red cell bursts. The structure of haemozoin comprises a polymer of haems linked between the central ferric ion of one haem and a carboxylate side-group oxygen of another. This structure does not form spontaneously from either free haem or haemoglobin under physiological conditions, and the biochemistry of its formation is unclear. Here we report the identification and characterization of a haem polymerase enzyme activity from extracts of Plasmodium falciparum trophozoites, and show that this enzyme is inhibited by quinoline-containing drugs such as chloroquine and quinine. This provides a possible explanation for the highly stage-specific antimalarial properties of these drugs.

Endogenous digitalis-like factors

The postulate of a natriuretic factor inhibiting the sodium pump in the kidney led to the detection of increased concentrations of endogenous digitalis-like factors in blood after salt loading, in essential hypertension, in pregnancy-induced hypertension and in chronic hypervolaemia. The recent isolation of ouabain or a close isomer thereof from human plasma and the demonstration of a compound similar if not identical to digoxin in adrenals and human urine shows that mammals like non-vertebrates and toads may synthesize cardiac glycosides in their adrenals and possibly in hypothalamus. The hypothalamus also forms other compounds of unknown structure which bind to the cardiac glycoside receptor site. The differential functions of endogenously formed ouabain and of a digoxin-like substance are unclear. The detailed knowledge of the physiological role of both endogenously formed cardiac glycosides in the regulation of blood pressure has still to be worked out.