Identification of a distinct sialic acid (KDN) and a KDN-specific aldolase in Pacific oyster

Sialic acids are a diverse family of acidic sugars typically found at the terminal positions of glycan chains, mediating key physiological and pathological processes across animals - particularly vertebrates - including cell signaling and host-pathogen interactions. The distribution of sialic acids in lower animals such as mollusks, however, remains largely unresolved. Here, we report the discovery of unconjugated 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN), a deaminated analogue of N-acetylneuraminic acid, in the muscle tissue of Pacific oysters (Magallana gigas). Using UPLC-ESI-MS/MS fingerprinting, we identified naturally occurring free KDN at a concentration of 1.2 ± 0.1 nmol/100 mg of oyster muscle tissue. To investigate the biosynthetic pathway, four candidate genes were identified in the M. gigas genome, and the corresponding recombinant proteins were expressed and characterized. Enzymatic assays revealed that one putative sialic acid aldolase (MgNPL) specifically catalyzes the cleavage of KDN into mannose and pyruvate. To our knowledge, this represents the first molecular evidence of KDN metabolism in mollusks and highlights both the unexpected conservation of substrate-specific aldolase activity and distinct sialic acid utilization mechanisms compared to vertebrates.