OVERT AND LATENT ACTIVITIES OF DIACYLGLYCEROL ACYLTRANSFERASE IN RAT-LIVER MICROSOMES - POSSIBLE ROLES IN VERY-LOW-DENSITY LIPOPROTEIN TRIACYLGLYCEROL SECRETION

The possibility that triacylglycerol (TAG) synthesis occurs on both as pects of the endoplasmic-reticular membrane during the process of inco rporation of TAG into secreted very-low-density lipoprotein (VLDL) [Za mmit (1996) Biochem. J. 314, 1-14] was investigated by measuring the l atency of diacylglycerol acyltransferase (DGAT) in microsomal fraction s obtained from rat liver homogenates. Permeabilization of microsomes with taurocholate resulted in the doubling of the activity, indicating that DGAT activities of approximately equal magnitude occur on either aspect of the microsomal membrane. The taurocholate concentrations re quired for exposure of the latent activity of DGAT were identical with those that resulted in the exposure of marker enzymes for the lumen o f the endoplasmic reticulum. Fractionation of the microsomes into smoo th and rough populations indicated that the distribution of overt and latent DGAT activities was the same throughout. The possibility that t aurocholate effects may result from non-specific activation of the ove rt enzyme was excluded by employing the channel-forming peptide alamet hicin to effect permeabilization, and by varying the mode of delivery of diacylglycerol substrate to the microsomal membranes. Permeabilizat ion using alamethicin gave a slightly higher latent/overt ratio for DG AT. The possible roles of overt and latent DGAT activities in the synt hesis and secretion of TAG by the liver are discussed.