The activities of ten enzymes involved in sialic acid metabolism were measured in colonic mucosal cells from rats and compared with those in liver. A methodology was devised that enabled all ten enzyme activities to be evaluated in a single rat colon preparation. Enzyme assays with radioactively labelled substrates were developed for maximum sensitivity, and the identification of substrates and products was carefully checked to assess the contribution of contaminants to enzyme reactions with low activity. The activities of most enzymes involved in the biosynthesis of N-acetyl-D-neuraminic acid (NeuAc) from UDP-N-acetyl-D-glucosamine were found to be more than 20-fold lower than those in liver. The activities of CMP-NeuAc synthase, N-acetyl-D-glucosamine 2-epimerase, N-acetyl-D-glucosamine kinase, sialyltransferase and sialidase were similar to or 2-4-fold lower than in liver. The biosynthesis of NeuAc via its 9-phosphate was demonstrated in the 100 000 g supernatant of colonic-cell homogenates by enzymic assay and precursor experiments with N-acetyl[14C]-mannosamine. No alternative route for NeuAc formation could be detected. The 100 000g supernatant fractions of liver, kidney and colonic mucosal cells utilized N-acetyl[14C]mannosamine with differing efficiencies. Radioactive products identified as sialic acid biosynthetic intermediates amounted to 49%, 0.04% and 5.6% of added precursor in liver, kidney and colon respectively. Catabolism of labelled precursor to non-hexosamine products was high in kidney and colonic mucosal-cell fractions.

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