In the development of proteins, aptamers, and molecular
imprints
for diagnostic purposes, a major goal is to obtain a molecule with
both a high binding affinity and specificity for the target ligand.
Cushing syndrome or Addison’s disease can be diagnosed by cortisol
level tests. We have previously characterized and solved the crystal
structure of an anti-cortisol (17) Fab fragment having a high affinity
to cortisol but also significant cross-reactivity to other glucocorticoids,
especially the glucocorticoid drug prednisolone. We used native mass
spectrometry (MS) to determine the binding affinities of nine steroid
hormones to anti-cortisol (17) Fab, including steroidogenic precursors
of cortisol. Based on the results, the number of hydroxyl groups in
the structure of a steroid ligand plays a key role in the antigen
recognition by the Fab fragment as the ligands with three hydroxyl
groups, cortisol and prednisolone, had the highest affinities. The
antibody affinity toward steroid hormones often decreases with a decrease
in the number of hydroxyl groups in the structure. The presence of
the hydroxyl group at position C11 increased the affinity more than
did the other hydroxyl groups at positions C17 or C21. The binding
affinities obtained by native MS were compared to the values determined
by surface plasmon resonance (SPR), and the affinities were found
to correlate well between these two techniques. Our study demonstrates
that native MS with a large dynamic range and high sensitivity is
a versatile tool for ligand binding studies of proteins.