Factors enhancing diacylglycerol acyltransferase activity in microsomes from cell-suspension cultures of oilseed rape

Several factors, including an unidentified endogenous component, were found to stimulate microsomal diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) from a microspore-derived cell-suspension culture of oilseed rape (Brassic a napus L. cv. Jet Neuf). At a concentration of 25 mM, MgSO4 and MgCl2 stim ulated microsomal DGAT 25- and 10-fold, respectively. ATP and CoA at concen trations of 2 and 1 mM stimulated the enzyme 2.4- and 12-fold, respectively , although the effects were lessened in the presence of higher Mg2+ concent rations. Although microsomal DGAT activity was increased only slightly by t he addition of exogenous sn-1,2-diacylglycerol to the reaction mixture, it was increased substantially by the addition of exogenous phosphatidate. sn- Glycerol-3-phosphate and other phospholipids tested did not have this stimu latory effect. DGAT activity did not decrease when microsomes were incubate d with ATP in the presence of the cytosolic fraction. This fraction, howeve r, contained a small organic compound(s) that stimulated microsomal DGAT ac tivity.