The specificity of sialyltransferase activity in smooth membrane fractions of embryonic chicken liver

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Abstract
Smooth membrane preparations of 13-day embryonic chicken livers, characterized by electron microscopy and marker enzyme analyses, have been found to contain sialyltransferase activity which displayed precise acceptor specificity. One sialyltransferase transferred N-acetylneuraminic acid (NANA) to galβ1 → 4glcNAcβ1 → R structures. Evidence based on competition studies suggests that a second enzyme is present transferring this sugar to a galβ1 → 3galNAcα1 → R structure. The enzyme capable of adding NANA to galβ1 → 4glcNAcβ1 → R structures has a pH optimum of 5.5, a temperature optimum of 30 °C, and half-saturating values of 17 μM for CMP-NANA and 180 μM for galactoside termini on desialyzed α1-acid glycoprotein. It is activated about 10-fold by Triton X-100, has no exogenous divalent cation requirement, and is inhibited by CTP, CDP, and CMP. The enzyme requires carbohydrate structures underlying the galβ1 → 4glcNAc terminus for maximal catalytic activity; the necessity of such precise specificities of sialyltransferases is discussed in the light of recent structural evidence for the carbohydrate moieties of several glycoproteins.

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