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Metabolism and Enzymology

Regulation by Phospholipids and Kinetic Studies of Plant Membrane-Bound UDP-Glucose Sterol -D-Glucosyl Transferase ¹

Laboratoire de Biochimie Végétale, Institut de Botanique, 28, rue Goethe, 67083 Strasbourg, Cédex, France

Solubilization and partial purification of the microsomal UDP-glucose sterol glucosyl transferase activity from maize coleoptiles by chromatography on DEAE-cellulose resulted in a highly delipidated (>95%) and inactive enzymic preparation. Addition of sterols revealed part of the activity and subsequent addition of phospholipids further increased the activity. Negatively charged phospholipids were shown to be by far the best activators. The purification step also produced the elimination of two interfering microsomal enzymic activities: UDPase and steryl glucoside acyl transferase. The removal of these two enzymic activities was a prerequisite for kinetic studies including product-inhibition studies, since the substrates of these two latter enzymes are the products of UDPG-SGTase activity. The results of the kinetic studies strongly suggest an ordered bi-bi mechanism for the glucosylation of sterols. Finally the effect of different phospholipids on the kinetic parameters of the reaction was studied. Both phosphatidylcholine and phosphatidylglycerol significantly decrease K_m-sterol (and not K_m-UDPglucose) and increase the reaction V_max. The decrease of K_m-sterol is similar with both phospholipids whereas the increase of V_max is much greater with phosphatidylglycerol than with phosphatidylcholine.

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