Catalyzes the reversible retro-aldol cleavage of 2-keto-3-deoxy-L-rhamnonate (KDR) to pyruvate and lactaldehyde. 2-keto-3-deoxy-L-mannonate, 2-keto-3-deoxy-L-lyxonate and 4-hydroxy-2-ketoheptane-1,7-dioate (HKHD) are also reasonably good substrates, although 2-keto-3-deoxy-L-rhamnonate is likely to be the physiological substrate. 2-dehydro-3-deoxy-L-rhamnonate = (S)-lactaldehyde + pyruvate Binds 1 Mg(2+) ion per subunit. Is more efficient when using Ni(2+) ion, although it is not likely to be the physiologically relevant active site metal. The catalytic efficiency observed with HKHD as substrate is 750-fold higher with Ni(2+) as cofactor than that with Mg(2+) as cofactor. 4-hydroxy-2-keto-5-phenyl-pentanoate and 4-hydroxy-2-keto-6-phenylhexanoate are not substrates, suggesting a requirement for an aliphatic or less bulky distal end of the substrate. Homohexamer. Belongs to the HpcH/HpaI aldolase family. KDR aldolase subfamily.