Catalyzes the isomerization between 2-isopropylmalate and 3-isopropylmalate, via the formation of 2-isopropylmaleate (Probable). Functions redundantly with LEUD1 in the methionine chain elongation pathway of aliphatic glucosinolate formation. (2R,3S)-3-isopropylmalate = (2S)-2-isopropylmalate a 2-(omega-methylsulfanyl)alkylmalate = a 2-(omega-methylsulfanyl)alkylmaleate + H2O 2-(3-methylsulfanyl)propylmalate = 2-(2-methylsulfanyl)propylmaleate + H2O a 3-(omega-methylsulfanyl)alkylmalate = a 2-(omega-methylsulfanyl)alkylmaleate + H2O 2-(2-methylsulfanyl)ethylmalate = 2-(2-methylsulfanyl)ethylmaleate + H2O 3-(2-methylsulfanyl)ethylmalate = 2-(2-methylsulfanyl)ethylmaleate + H2O 3-(3-methylsulfanyl)propylmalate = 2-(2-methylsulfanyl)propylmaleate + H2O Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 2/4. Heterodimer of the large LEUC/IIL1 subunit and the small LEUD (SSU1, SSU2 or SSU3) subunits. Expressed in vascular bundles of roots, cotyledons and rosette leaves (PubMed:24608865). Expressed in stem vascular bundles which branche off into lateral inflorescences (PubMed:24608865). Expressed in connective tissues in anthers (PubMed:24608865). In hypocotyls, expressed in parenchyma cells surrounding the vasculature (PubMed:32612621). In rosette leaves, expressed in phloem cells and cells close to the xylem along the vascular bundles (PubMed:32612621). In roots of adult plants, expressed in cells closely associated with the stele (PubMed:32612621). In flowering stalks, expressed in parenchyma cells associated with the phloem or the xylem (PubMed:32612621). No visible phenotype under normal growth conditions. Belongs to the LeuD family.