Catalyzes the dehydration of methylthioribulose-1-phosphate (MTRu-1-P) into 2,3-diketo-5-methylthiopentyl-1-phosphate (DK-MTP-1-P). Functions in the methionine salvage pathway, which plays a key role in cancer, apoptosis, microbial proliferation and inflammation. May inhibit the CASP1-related inflammatory response (pyroptosis), the CASP9-dependent apoptotic pathway and the cytochrome c-dependent and APAF1-mediated cell death. S-methyl-5-thio-D-ribulose 1-phosphate = 5-methylsulfanyl-2,3-dioxopentyl phosphate + H2O Binds 1 zinc ion per subunit. Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; L-methionine from S-methyl-5-thio-alpha-D-ribose 1-phosphate: step 2/6. Homotetramer. Interacts with APAF1. May interact with CASP1. Belongs to the aldolase class II family. MtnB subfamily.