Catalyzes the retro-aldol cleavage of 4-hydroxy-2-oxopentanoate to pyruvate and acetaldehyde. Is involved in the meta-cleavage pathway for the degradation of 2-aminophenol. (S)-4-hydroxy-2-oxopentanoate = acetaldehyde + pyruvate Belongs to the 4-hydroxy-2-oxovalerate aldolase family.