UDP-GlcNAc 2-epimerase/ManNAc kinase is the key enzyme of sialic acid biosynthesis in mammals. Its functional expression is a prerequisite for early embryogenesis and for the synthesis of several cell recognition motifs in adult organism. This bifunctional enzyme is involved in the development of different diseases like sialuria or hereditary inclusion body myopathy. For a detailed understanding of the enzyme, large amounts of the pure active protein are needed. Different heterologous cell systems were therefore analyzed for the enzyme, which was found to be functionally expressed in Escherichia coli, the yeast strains Saccharomyces cerevisiae and Pichia pastoris, and insect cells. In all these cell types, the expressed enzyme displayed both epimerase and kinase activities. In E coli, up to 2 mg protein/l cell culture was expressed, in yeast cells only 0.4 mg/L, while up to 100 mg/L, were detected in insect cells. In all three cell systems, insoluble protein aggregates were also observed. Purification from E coli resulted in 100 mug/L pure and structurally intact protein. For insect cells, purification methods were established which resulted in up to 50 mg/L pure, soluble, and active protein. In summary, expression and purification of the UDP-GlcNAc 2-epimerase/ManNAc kinase in Sf-900 cells can yield the milligram amounts of protein required for structural characterization of the enzyme. However, the easier expression in E coli and yeast provides sufficient quantities for enzymatic and kinetic characterization. (C) 2004 Elsevier Inc. All rights reserved.