Haptoglobin polymorphism and body iron stores

Abstract

In humans the iron status is influenced by environmental and genetic factors. Among them, the genetic polymorphism of the hemoglobin (Hb)-binding plasma protein haptoglobin (Hp) has been shown to affect iron turnover. The best known biological function of Hp is capture of free Hb in plasma to allow hepatic recycling of heme iron and to prevent kidney damage during hemolysis. In healthy males, but not in females, the Hp 2-2 phenotype is associated with higher serum iron, higher transferrin saturation, and higher ferritin than Hp 1-1 and 2-1. Moreover, serum ferritin correlates with monocyte L-ferritin content, which is also highest in Hp 2-2 subjects due to endocytosis of multimeric Hb-Hp 2-2 complexes by the recently identified Hb scavenger receptor CD163 in macrophages. This iron delocalization pathway, occurring selectively in Hp 2-2 subjects, has important biological and clinical consequences. The Hp polymorphism is related to the prevalence and the outcome of various pathological conditions with altered iron metabolism such as hemochromatosis, infections, and atherosclerotic vascular disease.