Associates with and regulates the activity of the sodium/potassium-transporting ATPase (NKA) which transports Na(+) out of the cell and K(+) into the cell. Reduces glutathionylation of the NKA beta-1 subunit ATP1B1, thus reversing glutathionylation-mediated inhibition of ATP1B1. Induces a hyperpolarization-activated chloride current when expressed in Xenopus oocytes. Regulatory subunit of the sodium/potassium-transporting ATPase which is composed of a catalytic alpha subunit, a non-catalytic beta subunit and an additional regulatory subunit. Interacts with catalytic alpha subunit ATP1A1. Also interacts with non-catalytic beta subunit ATP1B1. Interacts with the alpha1-beta1, alpha2-beta1 and alpha3-beta1 NKA isozymes. Glutathionylated. Belongs to the FXYD family.