Catalyzes the formation of proponate and pyruvate from (2R,3S)-2,3-dimethylmalate. Has no activity toward dimethylmaleate, malate, citramalate, isocitrate and citrate. (2R,3S)-2,3-dimethylmalate = propanoate + pyruvate Completely inhibited by propionic anhydride and by cystamine. Irreversibly inhibited by the mercapto reagents iodoacetate and iodoacetamide. Unaffected by hydroxylamine. Cofactor degradation; nicotinate degradation; propanoate and pyruvate from 6-hydroxynicotinate: step 8/8. Homotetramer. Belongs to the isocitrate lyase/PEP mutase superfamily.