Magnetic and natural circular dichroism of metalloporphyrin complexes of human and rabbit hemopexin.

Magnetic circular dichroism (MCD) spectra of several metalloporphyrin complexes of rabbit and human serum hemopexins in the spectral region of 300 to 650 nm and natural circular dichroism (CD) in the 300 to 450 nm region are reported. The MCD spectra of the heme (iron-protoporphyrin IX) complexes of both proteins were essentially identical suggesting similar iron coordination. The Soret region MCD spectrum of ferriheme . hemopexin has a shape and amplitude typical of other completely low spin (S = l/2) ferric hemeproteins, and the temperature dependence of the MCD intensity indicates that it is composed predominantly of Faraday C-type. terms. The visible region MCD spectrum of this complex closely resembles those characteristic of cytochrome b, and other bisimidazole-coordinated heme derivatives. Under aerobic conditions, heme. hemopexin is in the fully oxidized state. The ferroheme hemopexins also exhibit MCD spectra similar to that of ferrocytochrome b,, consistent with a low spin state and histidyl side-chain coordination of the heme iron in the reduced as well as in the oxidized state. The deuteroheme derivatives of rabbit hemopexin exhibit MCD spectra similar to those of the heme complex except for the expected slight differences in wavelength extrema, indicating that the vinyl side chains of protoporphyrin have little influence on the coordination. In contrast, the natural CD spectra of the heme complexes of rabbit and human hemopexin do not resemble the CD of cytochrome b,, reflecting differences in the crevice regions of the different hemeproteins. Furthermore, the CD spectra of the ferroheme complexes of rabbit and human hemopexin point to differences in the local environments of the heme chromophores.