Glycosylation by plasma membrane-bound glycosyl transferases or different turnover rates of plasma membrane compounds is regarded as responsible for the shorter half-life of plasma membrane-bound NANA

The degradation rates of N-acetylneuraminic acid (NANA) were investigated in plasma membranes and in the total protein of rat liver and Morris hepatoma 7777. The plasma membranes of Morris hepatoma 7777 could not be isolated by known methods useful for isolation of rat liver plasma membranes. Therefore, a modified method was developed. The specific activity of 5'-nucleotidase as a plasma membrane marker enzyme was enriched twelvefold relative to homogenate. N-Acetyl-D-mannosamine is the most suitable precursor of protein-bound NANA. When compared to the use of free NANA of a comparable specific radioactivity, the rate of incorporation is 6 times higher in liver and 20 times higher in hepatoma. †̃ “ C labeling in the acetyl group or in the hexose skeleton of N-acetyl-D-mannosamine resulted in identical measurement of the half-life of NANA, leading to a uniform turnover of the NANA molecule. There is no difference between the half-lives of NANA of the liver membranes and hepatoma membranes. The same applies to total liver and hepatoma protein. Yet, in both tissues, the half-life of plasma membrane-bound NANA is shorter (liver, 22.2 to 24 hr; host liver, 25.5 hr; Morris hepatoma 7777; 23.7 hr) than the half-life of total protein bound NANA (liver, 34 hr; host liver, 30.1 hr; Morris hepatoma 7777, 33.9 hr). This finding indicates that sialopro teins with considerably longer half-lives of NANA exist in subcellular structures other than plasma membranes. In plasma membranes, the half-lives of NANA were much shorter than the half-lives of amino acids when a non reutilizable amino acid precursor was used. It may be concluded that the mean turnover of NANA is more rapid than the turnover of amino acids in plasma membranes. Glycosylation by plasma membrane-bound glycosyl transferases or different turnover rates of plasma membrane compounds is regarded as responsible for the shorter half-life of plasma membrane-bound NANA.