Effect of vanadyl ions on calcineurin and its A subunit

Calcineurin (CN) exhibits a bimodal regulation by different concentrations of vanadyl ions (VO2+) in the presence of Mn2+. Low concentrations of VO2+ inhibit the enzyme, with 50 microM VO2+ completely inhibiting CN activity, while high concentrations, up to 500 microM VO2+, stimulate the CN activity. A similar bimodal regulation of CN was not observed with either calcium or vanadate under the same conditions. X-band electron spin resonance spectroscopy, used to study the binding of VO2+ to the catalytic subunit A of calcineurin, show that there are two kinds of binding sites in the A subunit.