Involved in both glucosinolate and leucine biosynthesis; catalyzes the oxidative decarboxylation step in both leucine biosynthesis (primary metabolism) and methionine chain elongation of glucosinolates (specialized metabolism) (PubMed:19674406, PubMed:19493961, 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). More active on 3-isopropylmalate and NAD(+) than towards D-malate (PubMed:19674406). (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; oxidation by CuCl(2) leads to a decreased activity. kcat is 37 min(-1) with 3-isopropylmalate as substrate (PubMed:20840499, PubMed:21697089). kcat is 51 min(-1) with 3-(2'-methylthio)ethylmalate as substrate (PubMed:21697089). Optimum pH is 7.6. Optimum temperature is 60 degrees Celsius. Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 3/4. Secondary metabolite biosynthesis. Homodimer. A number of isoforms are produced. According to EST sequences. Highly expressed in seedlings, leaves, stems and roots and, to a lower extent, in flowers, pollen and siliques. In young seedlings, expressed in all tissues except in the root tip. Later accumulates in cotyledons, newly emerging leaves and the lower region of roots. In rosettes, predominantly observed in the main veins of leaves. In flowers, present in petals, pistils and in the ends of young siliques. By wounding and jasmonic acid (MeJA). 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 (PubMed:19674406, PubMed:20840499). 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) (PubMed:20840499). In atimd1-1 and atipmdh1 mutants, reduced levels of methionine-derived glucosinolates (Met-GSLs) with long chains (C7-C8) and, to some extent, with short chains (C4-C6) (PubMed:19493961, PubMed:19674406, PubMed:21697089). Altered glucosinolate profile is restored by constructs expressing IPMDH2-L134F or IPMDH3-L133F mutants (PubMed:21697089). Belongs to the isocitrate and isopropylmalate dehydrogenases family.