Involved in leucine biosynthesis; catalyzes the oxidative decarboxylation step in leucine biosynthesis (primary metabolism) (PubMed:21697089). Catalyzes the oxidation of 3-carboxy-2-hydroxy-4-methylpentanoate (3-isopropylmalate, 3-IPM) to 3-carboxy-4-methyl-2-oxopentanoate. The product decarboxylates to 4-methyl-2 oxopentanoate (PubMed:15849421, PubMed:20840499). Required during pollen development and involved in embryo sac development (PubMed:20840499). (2R,3S)-3-isopropylmalate + NAD(+) = 4-methyl-2-oxopentanoate + CO2 + NADH Binds 1 Mg(2+) or Mn(2+) ion per subunit. Regulated by a thiol-based redox modification. kcat is 373 min(-1) with 3-isopropylmalate as substrate (at pH 7.5) (PubMed:20840499, PubMed:21697089). kcat is 274 min(-1) with 3-isopropylmalate as substrate (at pH 7.5 and 25 degrees Celsius) (PubMed:27137927). kcat is 1 min(-1) with 3-(2'-methylthio)ethylmalate as substrate (at pH 7.5) (PubMed:21697089). Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 3/4. Homodimer. Expressed at low levels in seedlings, cotyledons, hypocotyls, flowers, roots, pollen, leaves and stems. No discernible vegetative or reproductive phenotypes except a slight reduction of both male and female transmission efficiency, but decreased leucine biosynthetic enzyme activities and lower free leucine concentrations. The double mutant ipmdh2 ipmdh3 is lethal in male gametophytes (small aborted pollen grains abnormal in cellular structure, and arrested in germination) and had reduced transmission through female gametophytes (slow embryo sacs development). Belongs to the isocitrate and isopropylmalate dehydrogenases family.