= Methemoglobin
{title2=$\mathrm{MetHb}$}
\b[<Methemoglobin>] is a <hemoglobin> derivative with oxidized ferric <iron> at a <haem> site. The ferric site cannot perform the usual reversible oxygen binding of an available ferrous site. The distinction is one of <oxidation state>, not merely whether an oxygen molecule currently occupies the site.<Hemoglobin by Radiometer>[Additional manufacturer parameter page, Methemoglobin, What is MetHb?]{ref} An adult hemoglobin tetramer contains four haem sites, so site occupancy and the state of the whole tetramer must not be silently conflated.
<Deoxyhemoglobin> retains the capacity to bind oxygen at its available sites. A ferric site in <methemoglobin> instead requires chemical <reduction (chemistry)> to restore the corresponding ferrous state. Formation of <oxyhemoglobin> is therefore different from accumulating ferric haem. Changes in the surrounding protein also influence the properties of remaining sites; a site-count calculation alone is not a complete cooperative-binding model.
For an original capacity calculation, suppose a sample contains 100 micromoles of haem sites, with 20 micromoles ferric and the other 80 capable of reversible oxygen binding. The latter can bind at most 80 micromoles of oxygen in a one-molecule-per-site model. If 60 micromoles are oxygenated, the oxygenated fraction of all haem sites is $60/100=0.60$, while the occupied fraction of the stipulated capable sites is $60/80=0.75$. A denominator change produces different percentages without changing the sample.
Experimentally, <spectrophotometry> distinguishes derivatives through calibrated absorption spectra. A 1991 primary study measured human fetal and adult <oxyhemoglobin>, unoxygenated hemoglobin, <carboxyhemoglobin> and <methemoglobin> spectra over 450–1000 nm.<Absorption spectra of human fetal and adult oxyhemoglobin, de-oxyhemoglobin, carboxyhemoglobin, and methemoglobin by W. G. Zijlstra and others>[Abstract, measured millimolar absorptivities]{ref} The relevant inverse problem has the form
$$
A(\lambda)=\ell\sum_j\varepsilon_j(\lambda)c_j,
$$
where path length $\ell$, derivative concentrations $c_j$ and their calibrated absorption coefficients determine the modeled <absorbance>. <Co-oximetry> uses multiple spectral measurements to estimate such derivative fractions. A single colour observation cannot generally separate several unknown components.
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