N-Acetylneuraminic acid biosynthesis in rat liver and kidney

Rat liver and kidney tissue slices incubated withN-acetyl [³H]mannosamine incorporated radioactivity into free and boundN-acetylneuraminic acid and CMP-N-acetylneuraminic acid (CMP-NeuAc). Liver and kidney also incorporated radioactivity from intravenously injected [³H]ManNAc intoN-acetylneuraminic acid and CMP-NeuAc. From the decrease in the specific radioactivity of CMP-NeuAc after a single injection ofN-acetyl[³H]mannosamine the half-life of CMP-NeuAc was determined. From this half-life and the pool size of CMP-NeuAc a synthesis rate of CMP-NeuAc was calculated, being 1.2 nmol/min/g wet weight of kidney. In previous experiments a value of 1.0 nmol/min/g wet weight was determined for liver [Ferwerdaet al. (1983) Biochem J 216: 87–92]. The synthesis rate of CMP-NeuAcin vivo was in the same range as the synthesis rate calculated from the turnover of boundN-acetylneuraminic acid, which was 2.7 and 0.4 nmol/min/g wet weight for liver and kidney respectively.The assay conditions for UDP-N-acetylglucosamine 2-epimerase andN-acetylmannosamine kinase were adapted to measure low activitiesin vitro. It appeared that the kinase activity detected in kidney can synthesizeN-acetylmannosamine6-phosphate at a rate sufficient for the observed production ofN-acetylneuraminic acidin vivo. Also a low, but measurable activity of UDP-N-acetylglucosamine 2-epimerase was detected in kidneyin vitro, suggesting that the biosynthetic pathway ofN-acetylneuraminic acid in kidney is the same as in liver. The synthesis rate ofN-acetylneuraminic acid in liver determinedin vivo is approximately 12 times slower than the maximal potential rate calculated from the activities of theN-acetylneuraminic acid (precursor-) forming enzymes as detectedin vitro. This indicates that in liverin vivo the enzymes are working far below their maximal capacity.