Sialic acid as a tumor marker.

The term sialic acid is used to describe derivatives of neuraminic acid, where the amino group of neuraminic acid is substituted by either an acetyl or glycolyl group. The unique structural features of the molecule, which includes a negative charge owing to a carboxyl group, enables it to play a role in cellular functions, such as transport of positively charged compounds, cell-to-cell repulsion, influencing conformation of glycoproteins on cell membranes, and even masking antigenic determinants on receptor molecules. Focus on sialic acid as a tumor marker should be examined from the perspective of aberrant glycosylation in cancer cell membranes owing to activation of new glycosyl transferases that are characteristic of tumor cells, and the role played by sialic acid in tumor cell metastasis including increased capacity to adhere to vascular endothelium, and decreased capacity of cancer cells to be destroyed by host defence mechanisms. The high sensitivity of sialic acid as a tumor marker has been reported in a variety of cancerous conditions. Its specificity, however, is relatively low since there is also an increase in sialic acid-rich glycoproteins in inflammatory diseases. Sialic acid measurements, however, have value in monitoring cancer patients during treatment. A variety of methods are available for the measurement of both total and lipid associated sialic acids in serum or plasma. The newer high performance liquid chromatographic procedures can detect picogram levels of sialic acid and are relatively free of interferences seen with classical procedures.