Monocyte-macrophage ferric reductase activity is inhibited by iron and stimulated by cellular differentiation

The enzyme ferric reductase catalyses the reduction of Fe(III) as a prerequ isite to its transportation across the cell membrane. Duodenal mucosal biop sies from iron overloaded patients with genetic haemochromatosis (GH) have increased ferric reductase activity and iron absorption compared with contr ols, yet the GH mucosa is iron deficient. A similar GH-related iron deficie ncy is also seen in macrophages. The aim of this study was to investigate w hether macrophage ferric reductase activity is altered in GH, and to determ ine ferric reductase activity in monocytes and differentiated macrophages. The erythroleukaemic K562 cell line was studied as a clonal reference cell line. The basal K562 ferric reductase activity is characteristic of a membr ane bound enzyme, being both temperature and protease sensitive. Ferric red uctase activity was also demonstrated in human leucocyte, monocyte and macr ophage preparations. Assays of K562 and macrophage cell supernatants confir med that the ferric reductase activity was not due to a secreted factor. As say of ferric reductase in normalized-iron and iron-enriched (100 mu M ferr ic citrate) conditions showed no significant difference between Cys(282)-Ty r (Cys(282)-->Tyr) homozygous GH macrophages and Cys(282)-Tyr negative cont rol activities (P > 0.05). However, a 900 % increase in ferric reductase ac tivity was observed during monocyte to macrophage differentiation (P < 0.05 ), possibly reflecting the co-ordinate up-regulation of iron metabolism in these cells. The demonstration of approx. 25 % activity after macrophage di fferentiation at high free-iron concentrations compared with 'normalized' i ron is consistent with repression of human ferric reductase activity by iro n. The identification of the human ferric reductase gene and its protein wi ll ultimately provide insight into its regulation and role in mammalian iro n metabolism.