Cytoplasmic potassium channel subunit that modulates the characteristics of the channel-forming alpha-subunits. Modulates action potentials via its effect on the pore-forming alpha subunits (Probable). Promotes expression of the pore-forming alpha subunits at the cell membrane, and thereby increases channel activity (By similarity). Mediates closure of delayed rectifier potassium channels by physically obstructing the pore via its N-terminal domain and increases the speed of channel closure for other family members. Promotes the closure of KCNA1, KCNA2 and KCNA5 channels. Accelerates KCNA4 channel closure. Accelerates the closure of heteromeric channels formed by KCNA1 and KCNA4. Accelerates the closure of heteromeric channels formed by KCNA2, KCNA5 and KCNA6. Enhances KCNB1 and KCNB2 channel activity (By similarity). Binds NADPH; this is required for efficient down-regulation of potassium channel activity. Has NADPH-dependent aldoketoreductase activity. Oxidation of the bound NADPH strongly decreases N-type inactivation of potassium channel activity.