Evidence for indirect heme transfer from human methemoglobin to apoShp. Methemoglobin (total 5.8 μM heme) was reacted with 28 μM apoShp or 40 μM H64Y/V68F apomyoglobin in 20 mM Tris-HCl, pH 8.0, and 0.45 M sucrose, and absorbance time courses at 425 and 405 or 410 nm were recorded. (A) The normalized spectral changes Δ(A425-A405) (green curve) and Δ(A425-A410) (red curve) for the methemoglobin/apoShp and methemoglobin/apomyoglobin reactions, respectively. The black curves are the theoretical curves obtained by fitting the data to a two exponential equation. (B) Δ(A425-A405) of the methemoglobin/apoShp reaction does not fit a single exponential equation. The dashed and solid curves are respectively the observed data and theoretical curve from fitting of the data to a single exponential equation.