Host heme biosynthesis and degradation in schistosomiasis

In mice infected with Schistosoma mansoni, hepatic heme metabolism is markedly altered. The production of the immediate precursor, delta-aminolevulinic acid, is diminished, while the activity of the catabolic enzyme, heme oxygenase, is greatly increased. These changes are accompanied by a reduction in the cellular content of hemoproteins in various organs. Specifically, cytochrome levels in myocardial mitochondria are reduced, and liver cytochromes P-450 and b5 are also diminished. As a consequence of the latter, the microsomal oxidative enzyme activities, which are mediated by P-450, such as ethylmorphine N-demethylase and aniline hydroxylase, are considerably impaired. Barbiturate-induced sleeping time in mice heavily infected with schistosomes was found to be significantly prolonged. A green discoloration of the liver and spleen seen in advanced murine schistosomiasis is not likely to be due to the production of abnormal pyrrolic pigments, since hemoglobin heme was found to be degraded via the usual catabolic pathways to physiological bile pigments. Total serum iron was found to be increased by 100% in schistosome-infected mice. Serum unsaturated iron binding capacity was, however, not increased significantly. Demonstration that the activities of enzymes of heme metabolism, which are known to be regulated by heme and metal ions, are altered in the host as a consequence of the parasitism suggests that these perturbations may be mediated by heme or its iron released by the digestion of erythrocytes by schistosomes.