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). (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 543 min(-1) with 3-isopropylmalate as substrate (PubMed:20840499, PubMed:21697089). kcat is 1 min(-1) with 3-(2'-methylthio)ethylmalate as substrate (PubMed:21697089). Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 3/4. Homodimer. Mostly expressed constitutively at high levels in seedlings, cotyledons, hypocotyls, flowers, pollen and leaves and, to a lower extent, in roots and stems. Transactivated by MYB28. No discernible vegetative or reproductive phenotypes, but decreased leucine biosynthetic enzyme activities and lower free leucine concentrations. Belongs to the isocitrate and isopropylmalate dehydrogenases family.