Recently, we have shown that alpha 2,8-linked disialic acid (diSia) residue occurs in glycoproteins more frequently than ever recognized (Sato, C., Fukuoka, H., Ohta, K., Matsuda, T., Koshino, R., Kobayashi K., Troy, F. A., II, and Kitajima, K. (2000) J. Biol. Chem. 275, 15422--15431). In the course of identification of the diSia-containing glycoproteins in mammals, the 30-kDa glycoprotein was found in bovine serum. The 30-kDa glycoprotein was shown to be the bovine adipo Q, an adipocyte-specific protein, based on the partial amino acid sequences and the immuno-cross-reactivity with the recombinant mouse adipo Q. The bovine adipo Q was shown to have no N-linked but O-linked glycan(s) containing the diSia epitope, Neu5Ac alpha 2-->8Neu5Ac alpha 2-->3Gal. Furthermore, the diSia epitope was also found in the mouse adipo Q in serum as well as in the 3T3-L1 cells that are fully differentiated into adipocytes. Notably, among the known alpha 2,8-sialyltransferases, only the alpha 2,8-sialyltransferase III mRNA was detected in the 3T3-L1 cells at any stages of differentiation, and the recombinant alpha 2,8-sialyltransferase III could sialylate the purified bovine adipo Q. Thus, this study clearly provides the new findings that adipo Q is the diSia-containing glycoprotein and a physiological substrate of alpha 2,8-sialyltransferase III, whose substrates have not been identified so far.