Cleaves both heparin and heparan sulfate glycosaminoglycans through a beta-elimination mechanism. Cleaves heparin at alpha-D-GlcNp2S6S(1->4) alpha-L-IdoAp2S and heparan sulfate at alpha-D-GlcNp2Ac(or 2S)6OH(1->4)beta-D-GlcAp. Elimination of sulfate, appears to act on linkages between N-acetyl-D-glucosamine and uronate. Product is an unsaturated sugar. Eliminative cleavage of polysaccharides containing (1->4)-linked D-glucuronate or L-iduronate residues and (1->4)-alpha-linked 2-sulfoamino-2-deoxy-6-sulfo-D-glucose residues to give oligosaccharides with terminal 4-deoxy-alpha-D-gluc-4-enuronosyl groups at their non-reducing ends. Homodimer. Belongs to the polysaccharide lyase 12 family.