Different target specificities of haptoglobin and hemopexin define their sequential role as primary and backup protective systems against hemoglobin toxicity

ABSTRACT: Hemoglobin (Hb) released from red blood cells during hemolysis is a trigger of oxidative vascular damage with endothelial cells being a primary target. The Hb and heme scavenger proteins haptoglobin (Hp) and hemopexin (Hx) have been characterized as a sequential defense system with Hp as the primary protector and Hx as a backup when all Hp is depleted during more severe or prolonged hemolysis. The paradigm of sequential protection by the two scavengers is based on observational patient data proposed by Mueller-Eberhardt. In this study we present a mechanistic rationale for these clinical observations using a novel in vitro model of Hb triggered endothelial damage. We identified oxyHb(Fe2+) transformation to ferric Hb(Fe3+), free heme transfer from ferric Hb(Fe3+) to lipoprotein and subsequent oxidative reactions in the lipophilic phase. The accumulation of toxic lipid peroxidation products liberated during oxidation reactions lead to endothelial damage characterized by a specific gene expression pattern, reduced cellular ATP and monolayer disintegration. Quantitative analysis of key biochemical and functional parameters allowed us to precisely define the mechanisms and concentrations required for Hp and Hx to prevent this toxicity. In the case of Hp we defined an exponential relationship between Hp availability relative to oxyHb(Fe2+) and related protective activity. This exponential relationship demonstrates that large Hp quantities are required to prevent Hb toxicity. In contrast, the linear relationship between Hx concentration and protective activity allows for significant protection by the backup scavenger during conditions of large excess of free oxyHb(Fe2+) that occurs when all Hp is consumed. The diverse protective function of Hp and Hx in this model can be explained by the different target specificities of the two proteins.

ORGANISM(S): Homo sapiens

PROVIDER: GSE66283 | GEO | 2018/02/25

REPOSITORIES: GEO