Incorporation of Sialic Acid into Sialidase-Treated Apolipoprotein of Human, Very Low Density Lipoprotein by a Pork Liver Sialyltransferase

Abstract

Pork liver microsomes are capable of catalyzing the incorporation of N-¹⁴C-acetylneuraminic acid from CMP-N-¹⁴C-acetylneuraminic acid into a sialidase-treated apolipoprotein (apoLP-Ala₁) prepared from human, very low density lipoprotein and into sialidase-treated ovine submaxillary mucin. The pH optimum for the lipoprotein sialyltransferase is 5.8, and the K_m values for CMP-N-acetylneuraminic acid, sialidase-treated apoLP-Ala₁ and sialidase-treated ovine submaxillary mucin are, respectively, 0.04 mM, 0.04 mM, and 10 mM. Mixed substrate experiments indicate that the same sialyltransferase acts on both lipoprotein and mucin acceptors but that this enzyme is different from a previously described pork liver sialyltransferase which incorporates N-acetylneuraminic acid into linkage with the terminal galactose residues of sialidase-treated α₁-acid glycoprotein. The radioactive product of the lipoprotein sialyltransferase releases, on treatment with alkaline borohydride, a reduced oligosaccharide tentatively identified as N-¹⁴C-acetylneuraminyl-N-acetylgalactosaminitol.