Cryptophane xenon-129 nuclear magnetic resonance biosensors targeting human carbonic anhydrase.

(129)Xe NMR biosensors are promising agents for early disease detection, especially when their interactions with target biomolecules can perturb (129)Xe chemical shifts well beyond the typical field inhomogeneity of clinical MRI. We introduce human carbonic anhydrase (CA) as a single-binding-site enzyme for studying xenon biosensor-protein interactions. A xenon-binding cryptophane was substituted with linkers of varying lengths to p-benzenesulfonamide to yield nondiastereomeric biosensors with a single (129)Xe NMR resonance. X-ray crystallography confirmed binding of the eight-bond-linked biosensor containing a single xenon atom in the CAII active site. Biosensor dissociation constants (K(d) = 20-110 nM) were determined by isothermal titration calorimetry (ITC) for isozymes CA I and II. The biosensor-CA complexes yielded "bound" hyperpolarized (129)Xe NMR resonances of narrow line width that were shifted by 3.0-7.5 ppm downfield, signifying much larger shifts than seen previously. Moreover, isozyme-specific chemical shifts clearly differentiated CA I and II, despite their similar structures. Thus, xenon biosensors may provide a powerful strategy for diagnosing human diseases characterized by the upregulation of specific CA isozymes and other protein biomarkers.