Biosynthesis of N ‐Acetylneuraminic Acid in Morris Hepatomas

The biosynthesis of N ‐acetylneuraminic acid was studied in liver, neonatal liver, regenerating liver and Morris hepatomas 7777, 5123 TC, 5123 C, 5123 D, 7800, 7795, 9121 and 9618 A. The following three parameters were investigated: the biosynthesis of N ‐acetylneuraminic acid from d ‐[1‐ 14 C]glucosamine and d ‐ N ‐acetyl‐[U‐ 14 C]mannosamine; the content of free and protein bound N ‐acetylneuraminic acid and of CMP‐ N ‐acetylneuraminic acid; the activity of the key enzyme (UDP‐ N ‐acetylglucosamine‐2′‐epimerase).1 The biosynthesis of N ‐acetylneuraminic acid was investigated in vivo by labelling experiments using [1‐ 14 C]glucosamine and N ‐acetyl‐[U‐ 14 C]mannosamine as precursors revealing the same metabolic route in liver and hepatoma. The biosynthesis of N ‐acetylneuraminic acid deriving from [1‐ 14 C]glucosamine in vivo is diminished to about 15% in hepatoma compared to liver, whereas the synthesis of UDP‐ N ‐acetylglucosamine is not altered. The uptake of N ‐acetyl‐[U‐ 14 C]mannosamine and its conversion to N ‐acetylneuraminic acid in liver and hepatoma is demonstrated. The synthesis of free N ‐acetylneuraminic acid deriving from N ‐acetyl‐[U‐ 14 C]‐mannosamine is not changed in hepatoma. The incorporation of this precursor into protein‐bound N ‐acetylneuraminic acid of hepatoma is four‐times higher. 2 The content of free and protein bound N ‐acetylneuraminic acid and CMP‐ N ‐acetylneuraminic acid have been measured in livers and hepatomas by isotope dilution technique. About 90% of total N acid content in both livers and hepatomas is protein‐bound. In Morris hepatomas the content of protein‐bound N ‐acetylneuraminic acid is elevated (between 5.93 and 8.38 nmol/mg protein) compared to liver (4.50 ± 0.52 nmol/mg protein). However the content of free. N ‐acetylneuraminic acid including CMP‐ N ‐acetylneuraminic acid is diminished in Morris hepatomas (between 108 and 164 nmol/g hepatoma in comparison to 196 ± 28 nmol/g liver), whereas the content of CMP‐ N ‐acetylneuraminic acid alone is not greatly changed (31.4 ± 8.5 nmol/g Morris hepatoma 9121 in comparison to 40.8 ± 10.1 nmol/g liver). 3 The activity of UDP‐ N ‐acetylglucosamine‐2′‐epimerase of all Morris hepatomas investigated was diminished to between 6% and 13% compared to that of host liver. In other fast growing liver tissue (neonatal and regenerating liver) this enzyme activity was not diminished but slightly increased showing that the decreased enzyme activity is a characteristic feature of Morris hepatomas.