Modification of Rat Hemopexin Properties upon Heme Binding

Abstract

Rat hemopexin and its complex with heme had the same Stokes'' radius, 3.9 nm, as determined by gel filtration. Therefore no polymerization occurs as a result of heme binding. The conformational parameters calculated from circular dichroism spectra indicate that hemopexin and its complex consist of 20% .beta. sheet and mainly of disordered structure. No change of the secondary structure is therefore observed upon heme binding. Hemopexin reveals 5 bands by analytical electrofocusing with pI [isoelectric point] ranging from 5.5-5.95. This microheterogeneity is not due to sialic acid differences between the variants. Upon heme binding the pI of the variants decrease to lower values (from 4.8 to 5.25). This decrease in the pI value of hemopexin is thought to modify the tertiary structure through charge effects and may allow the binding of the heme-hemopexin complex to the hepatocytes.