The modulation of bovine milk D-galactosyltransferase by various phosphatidylethanolamines

To investigate the possible role of nonbilayer phases in the modulation of glycosyltransferase activity, bovine milk D-galactosyltransferase has been studied in phosphatidylethanolamine (PE) membranes, including soybean PE, egg PE, PE prepared by transphosphatidylation of egg PC, bovine brain PE, plasmalogen PE, and DPPE. The gel-to-liquid crystalline transition (TC) and the lamellar-to-hexagonal transitions (TH) are known for most of the PE compounds. The lower the TC (or TH) value, the greater the stimulation of galactosyltransferase activity in both the lactose- and N-acetyllactosamine-synthetase reactions. No correlation was found between either TC or TH value and the break in the Arrhenius plots for the N-acetyllactosamine synthetase. In membranes consisting of mixtures of PE with PC, the dominant effect was that of PC. The stimulation of activity in the mixed-lipid systems was never greater than that produced by PC alone, therefore the enzyme showed a definite preference for PC in the mixtures.