Catalyzes the cleavage of N-acetylneuraminic acid (sialic acid) to form pyruvate and N-acetylmannosamine via a Schiff base intermediate. It prevents sialic acids from being recycled and returning to the cell surface. Involved in the N-glycolylneuraminic acid (Neu5Gc) degradation pathway (By similarity). aceneuramate = N-acetyl-D-mannosamine + pyruvate Amino-sugar metabolism; N-acetylneuraminate degradation. Homotetramer. Belongs to the DapA family. NanA subfamily.