Variations in sialic acid content of gamma-glutamyltransferase: a consequence for immunochemical determinations?

Using specific antibodies against the human kidney enzyme, gamma-glutamyltransferase (GGT, EC 2.3.2.2) was assayed from human kidney and serum by electroimmunodiffusion. Determination of the enzyme by such a method was highly influenced by the sialic acid content of the molecule. The peaks corresponding to the sialylated GGT were higher than those corresponding to the neuraminidase-treated enzyme. In contrast, sialylation of the protein had no influence on the results observed when measuring the enzyme by radial immunodiffusion. Moreover, immunoprecipitation curves of both sialylated and neuraminidase-treated samples were identical. The varying degrees of sialylation of GGT occurring under physiological or pathological conditions are known to be partly responsible for the heterogeneity of the enzyme in organs and biological fluids. Therefore, determination of the enzyme by electroimmunodiffusion may be hazardous.