Synthesis of N-acetylneuraminic acid and of CMP-N-acetylneuraminic acid in the rat liver cell

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Abstract
Adult male rats, under starving and normal conditions, were injected i.v. with N-acetyl[3H]mannosamine and after various time intervals the specific radioactivities of free N-acetylneuraminic acid (NeuAc) and CMP-N-acetylneuraminic acid were determined in the liver. The specific radioactivity of free NeuAc was high even within 20s after injection; the maximum was reached between 7 and 10 min. The specific radioactivity of CMP-NeuAc showed a lag phase of .apprx. 1 min. Thereafter it increased quickly and rose above the specific radioactivity of free NeuAc, reaching a maximum about 20 min after injection. These results point to a channeling of the newly synthesized NeuAc molecules into a special compartment, from which they are preferentially used by the enzyme CMP-sialic acid synthetase. The cytosolic enzyme N-acetylneuraminic acid 9-phosphate phosphatase may be working concert with the nuclear localized enzyme CMP-N-acetylneuraminic acid synthetase. Incorporation of radioactive sialic acid into sialoglycoproteins in liver occurred 2 min after injection, and after 10 min bound radioactivity began to appear in the circulation, indicating a transport time of 8 min of sialoglycoproteins from the point of attachment of sialic acid to the point of excretion.