GO:0003957

Catalysis of the reaction: NADPH + H+ + NAD+ = NADP+ + NADH + H+, driving the transfer of a solute or solutes from one side of a membrane to the other. In the course of the reaction (left to right) one H atom is transferred from inside the cell to outside. The reaction is B-specific (i.e. the pro-S hydrogen is transferred from the 4-position of reduced nicotinamide cofactor) with respect to both NAD+ and NADP+.