Monoterpene synthase that catalyzes the specific reduction of the 4,8-double bond of (+)-pulegone to produce both (-)-menthone and (+)-isomenthone in a 70:30 ratio. Unable to utilize either (-)-isopiperitenone or (+)-cis-isopulegone, or to catalyze the reverse reaction with (-)-menthone or (+)-isomenthone. Has an absolute requirement for NADPH. (2R,5R)-isomenthone + NADP(+) = (R)-pulegone + H(+) + NADPH (1R,4S)-menthone + NADP(+) = (R)-pulegone + H(+) + NADPH Not inhibited by (+)-menthofuran. Optimum pH is 5.0. Secondary metabolite biosynthesis; terpenoid biosynthesis. Down-regulated by (+)-menthofuran. Belongs to the NADP-dependent oxidoreductase L4BD family.