Some aspects of glycosyltransferase activities in lung tissue

In our laboratory high sialyl- and galactosyltransferase activities were observed in lung homogenate.A differing behaviour using the detergent Triton X-100 was also observed for these enzymes. Following this the various properties and kinetic parameters of the two glycosyltransferases were studied in detail in lung microsomes. The optimum pH (6.8), optimum exogenous acceptor protein requirements (DS- and DSG- fetuin, 250 μg) and a linearity with time and protein concentration were established for sialyl- and galactosyltransferase. In addition, galactosyltransferase required Mn²⁺ (12.5mM) and ATP(2mM) for optimum activity. Sialyltransferase catalyzed the transfer of 2.3 nanomoles sialic acid to fetuin acceptor per mg protein per hour, and its apparent Km for CMP-sialic acid was 0.33mM. Galactosyltransferase catalyzed the transfer of 14.0 nanomoles of galactose to fetuin acceptor per mg protein per hour, and its apparent Km for UDP-galactose was 0.053mM. -- Inclusion of the detergent Triton X-100 caused an 8-fold increase in the activity of galactosyltransferase and the Vmax rose 6—fold, from 0,33 to 2.0. Sialyltransferase, in contrast, showed no dose—dependent response to Triton X-100 and was virtually unresponsive to Triton even in small dose. Little change in the Vmax of this enzyme in the presence of Triton was seen. Lysolecithin caused a specific activation of galactosyltransferase when compared to other lipid species added in a quantity considered to be physiological. Sialyltransferase, however, showed a general inhibitory response to all lipid classes tested. An additive effect was found on the glycosyltransferases when Triton and lipid were added on an equimolar basis. When the Triton dose was much greater than that of the lipid, the effects of the latter were masked by Triton. The one exception to this trend was found with LysoPA (oleoyl) whose effect on either enzyme's activity was not completely masked by the Triton. -- An enrichment of both sialyl- and galactosyltransferase was obtained from purified lung surfactant and lung lamellar bodies. During the isolation of lung surfactant, sialyl-transferase was found in soluble form and galactosyltransferase was localized in membrane-rich fractions. Major proteins of molecular weights 64,000, 35,000, and 23-28,000 were identified in lung surfactant and lamellar body fractions, as revealed by SDS-gel electrophoresis. -- A difference in the membrane localization and soluble nature of sialyl- and galactosyltransferase in lung tissue is discussed, with specific reference to the role of lysolecithin in this system. The role of glycosyltransferases found in lung surfactant and lamellar bodies may also involve membrane lipid changes in affecting the function of these enzymes. Possible relationships between glycosyltransferases and membrane phospholipids are discussed.