Apomyoglobin Sequesters Heme from Heme Bound Aβ Peptides

Abstract

A combination of absorption, electron paramagnetic resonance (EPR), and resonance Raman (rR) spectroscopy has been used to study the interaction of heme-Aβ and apomyoglobin (apoMb). The absorption spectrum of oxidized heme bound Aβ, characterized by a split Soret band at 364 and 394 nm, shifts to 408 nm on incubation with apoMb, characteristic of Myoglobin (Mb). The ν4, ν3, and ν2 bands in the rR spectrum of heme-Aβ are observed at 1376, 1495, and 1570 cm(-1), which shift to 1371, 1482, and 1563 cm(-1), respectively on incubating with apoMb, implying formation of Mb. Similarly, heme transfer from reduced heme-Aβ to apoMb resulting in the formation of deoxyMb was also observed. Thus, spectroscopic data show that apoMb can sequester heme from heme-Aβ complexes both in oxidized and in reduced forms. Heme uptake by apoMb from native heme-Aβ(1-40) and Aβ(1-16) in both oxidized and reduced forms follow a biphasic reaction kinetics likely representing heme transfer from two dominating conformers of heme-Aβ in solution. The rate constants for the two steps involved in heme uptake by apoMb from heme-Aβ(1-40) are 11.5 × 10(4) M(-1) s(-1) and 7.5 × 10(3) M(-1) s(-1) while from heme-Aβ(1-16) are 6.0 × 10(4) M(-1) s(-1) and 7.5 × 10(3) M(-1) s(-1). The rate constants for heme uptake by apoMb from reduced heme-Aβ(1-40) are 3.7 × 10(4) M(-1) s(-1) and 6.8 × 10(3) M(-1) s(-1) while for reduced heme-Aβ(1-16) are 2.0 × 10(4) M(-1) s(-1) and 6.0 × 10(3) M(-1) s(-1). The heme uptake from heme-Aβ by apoMb leads to a dramatic reduction of PROS generation by the reduced heme-Aβ complexes.