Hypoxia is a common characteristic of locally advanced tumors and cancer cells of high invasiveness and metastasis. We first demonstrate that hypoxic culture of human cancer HeLa, LS174T, and Caco-2 cells enhances the de novo synthesis of free sialic acids including deaminoneuraminic acid (Kdn), a unique sialic acid. Hypoxic cultures enhance expression of the N-acetylneuraminic acid (Neu5Ac) 9-phosphate synthetase (NPS) and phosphomannoisomerase (PMI) mRNAs and their enzymatic activities. In addition, incorporation of mannose (Man) into the cells is also enhanced. The elevated NPS activity facilitates the synthesis of Kdn as well as Neu5Ac, a typical sialic acid in human. The increased PMI activity, together with the enhanced Man-incorporation ability, leads to the increase of mannose 6-phosphate, which is a substrate of NPS, and subsequently results in the increased expression of Kdn, but not Neu5Ac. Kdn may thus be a target for diagnosis of cancers. (c) 2007 Elsevier Inc. All rights reserved.