Figure 3.

CMP-Neu5Ac inhibition of UDP-GlcNAc 2-epimerase activity. The substrate of epimerase is [³H]UDP-GlcNAc, and the product is [³H]ManNAc. [³H]GlcNAc represents a degradation product of [³H]UDP-GlcNAc, not a product of epimerase (Seppala et al. 1992). A, Epimerase activity in normal fibroblasts. Significant ManNAc is formed in the absence of CMP-Neu5Ac (left), and 100 mM CMP-Neu5Ac inhibits ManNAc formation by 95% (right). B, Fibroblast epimerase activity in patient 1. CMP-Neu5Ac, 100 mM (right), did not inhibit ManNAc production at all compared with ManNAc production in the absence of CMP-Neu5Ac (left).