Catalysis of the reaction: an N-acetylneuraminate glycoside = 2,7-anhydro-alpha-N-acetylneuraminate + an alpha-sialyl group. This reaction is the elimination of alpha-sialyl groups in N-acetylneuraminic acid glycosides, releasing 2,7-anhydro-alpha-N-acetylneuraminate.
glycoconjugate sialyl-lyase (2,7-cyclizing) activity