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The first two steps in mammalian biosynthesis of Nacetylneuraminic acid, an important carbohydrate moiety in biological recognition systems, are performed by the bifunctional enzyme UDPNacetylglucosamine 2-epimerase/Nacetylmannosamine kinase. A subclone of the human B lymphoma cell line BJAB K20, lacking UDPNacetylglucosamine 2- epimerase/Nacetylmannosamine kinase mRNA as well as epimerase activity, displayed hyposialylated, functionally impaired cell surface glycoconjugates. Here we show that this cell line surprisingly still retains Nacetylmannosamine kinase activity. A gel filtration analysis of BJAB K88 control cells, which express UDPNacetylglucosamine 2-epimerase/Nacetylmannosamine kinase, revealed two Nacetylmannosamine kinase activity peaks, one coeluting with UDPNacetylglucosamine 2-epimerase activity and one coeluting with Nacetylglucosamine kinase. For this enzyme previous studies already showed ManNAc kinase activity in vitro. In contrast, the hyposialylated BJAB K20 subclone displayed only the Nacetylmannosamine kinase peak, comigrating with Nacetylglucosamine kinase. The CMPNacetylneuraminic acid content of both K88 and K20 cells and the sialylation of cell surface glycoconjugates of K20 cells could be significantly increased by supple menting the medium with Nacetylmannosamine. This Nacetylmannosamineinduced increase was drastically reduced by cosupplementation with Nacetylglucosamine only in K20 cells. We therefore propose the phosphorylation of Nacetylmannosamine as a hitherto unrecognized role of Nacetylglucosamine kinase in living cells.

Biosynthesis of N-Acetylneuraminic Acid in Cells Lacking UDP-N-Acetylglucosamine 2-Epimerase/ N-Acetylmannosamine Kinase