Binding of vanadium (IV) to the phosphatase calcineurin

X-band electron spin resonance spectroscopy was used to study the binding of vanadium (IV), or vanadyl, to the brain serine/threonine phosphatase-2B, calcineurin. Spectra were determined on frozen solutions of vanadyl and calcineurin at pH 7.4 in the presence of 20% (v/v) glycerol. The binding of vanadyl to the enzyme was established, and the data suggested the presence of two classes of sites, the higher affinity class of which contained two binding sites for vanadyl. The calcium-binding B subunit of the heterodimeric protein was also shown to bind vanadyl. The holoprotein appeared to be stabilized by vanadyl, and vanadyl enhanced enzymatic activity when assayed with or without calmodulin in the absence of calcium.