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Effect of phosphatidylinositol and phosphatidylserine on membrane-bound galactosyltransferase.

Membrane-bound galactosyltransferase is solubilized and activated by exogenous lysolecithin or Triton X-100. A study on the effect of different phospholipids on the lysolecithin-solubilized enzyme showed that two charged phospholipids (i.e., phosphatidylinositol and phosphatidylserine) inhibited the membrane-bound enzyme in the presence of a wide range of lysolecithin concentration (up to 6 mumol/mg protein). In contrast, these phospholipids produced a biphasic effect on the enzyme solubilized with Triton X-100. In lower concentration of Triton (up to 2 mumol/mg protein), the charged phospholipids somewhat reduced the enzyme activity but a reversal of this effect was observed when Triton concentration was gradually raised (from 2 to 8 mumol/mg protein). This biphasic effect of the phospholipids was also demonstrated on purified membrane-bound galactosyltransferase in presence of low and high concentration of Triton. Electron microscopic evidence suggested that an increased concentration of phosphatidylinositol prevented membrane solubilization by lysolecithin or retained the membrane vesicular organization concurrent with a restraining effect on the enzyme. The results lend support to the hypothesis that the phospholipid microenvironment of the membrane may exert a control on the membrane-bound glycosyltransferases.