Macrobrachium borellii hepatopancreas contains a mitochondrial glycerol-3-phosphate acyltransferase wich initiates triacylglycerol biosynthesis

PELLON-MAISON M; GARCIA C F; CATTANEO E.R; COLEMAN R.A; GONZALEZ-BARÓ M. R

LIPIDS

AOCS, Champaign IL

Año: 2009 p. 337 - 344

0024-4201

     In mammals, four isoforms of glycerol-3-phosphate acyltransferase (GPAT, EC 2.3.1.15) have been described: GPAT1 and 2 are located on the outer mitochondrial membrane and GPAT 3 and 4 are in the endoplasmic reticulum (ER).  Although ER-GPATs account for the highest specific activity in mammalian tissues, it appears that GPAT1 may be the first-appearing acyltransferase in evolution.  In E. coli and other bacteria, the only protein that expresses GPAT activity is plsB, a protein with a molecular mass similar to that of rat GPAT1 and 30% amino acid identity. We report that triacylglycerol (TAG) synthesis in the crustacean Macrobrachium borellii hepatopancreas depends solely on a mitochondrial GPAT.      Hepatopancreas is an organ that has a high-TAG biosynthetic activity, and exerts functions analogous to both mammalian liver and adipose tissue with TAG stores of 80% of total lipids. In hepatopancreas mitochondria, we identified both GPAT activity and protein similar to mammalian GPAT1.  The activity was resistant to inactivation by SH-reactive substances, it was activated by polymixin-B, and its substrate preference was for palmitoyl-CoA.  Reaction products were similar to those of mammalian GPAT1.  We also visualized a 70-kDa protein band reactive to anti-rat liver GPAT1 antibody. Surprisingly, we did not detect GPAT activity in hepatopancreas microsomes higher the activity consistent with mitochondrial contamination in the microsomal fraction, even though these microsomes can synthesize TAG from [14C]palmitate.  TAG synthesis in microsomes is not dependent on the presence of glycerol-3 phosphate as a substrate, and the presence of Mg++ and dithiothreitol in the reaction mixture altered the reaction products and the extent of esterification of palmitate into glycerolipids.  We conclude that this crustacean model is unique in that the first step in the de novo biosynthetic pathways for glycerolipids is carried out exclusively in mitochondria and that ER completes the synthesis.