Regulatory subunit of the voltage-gated potassium (Kv) channels composed of pore-forming and potassium-conducting alpha subunits and of regulatory beta subunits. The beta-1/KCNAB1 cytoplasmic subunit 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 inactivation of KCNA1, KCNA2, KCNA4, KCNA5 and KCNA6 alpha subunit-containing channels. Displays nicotinamide adenine dinucleotide phosphate (NADPH)-dependent aldoketoreductase activity by catalyzing the NADPH-dependent reduction of a variety of endogenous aldehydes and ketones. The binding of NADPH is required for efficient down-regulation of potassium channel activity. Oxidation of the bound NADPH restrains N-terminal domain from blocking the channel, thereby decreasing N-type inactivation of potassium channel activity 