Catalyzes the terminal and only committed step in triacylglycerol synthesis by using diacylglycerol and fatty acyl CoA as substrates. In contrast to DGAT2 it is not essential for survival. May be involved in VLDL (very low density lipoprotein) assembly. In liver, plays a role in esterifying exogenous fatty acids to glycerol. Functions as the major acyl-CoA retinol acyltransferase (ARAT) in the skin, where it acts to maintain retinoid homeostasis and prevent retinoid toxicity leading to skin and hair disorders.
J. Biol. Chem. 273, 26765-26771 (1998)[PubMed:9756920]
The enzyme acyl coenzyme A:cholesterol acyltransferase 1 (ACAT1) mediates sterol esterification, a crucial component of intracellular lipid homeostasis. Two enzymes catalyze this activity in Saccharomyces cerevisiae (yeast), and several lines of evidence suggest multigene families may also exist in mammals. Using the human ACAT1 sequence to screen data bases of expressed sequence tags, we identified two novel and distinct partial human cDNAs. Full-length cDNA clones for these ACAT related gene products (ARGP) 1 and 2 were isolated from a hepatocyte (HepG2) cDNA library. ARGP1 was expressed in numerous human adult tissues and tissue culture cell lines, whereas expression of ARGP2 was more restricted. In vitro microsomal assays in a yeast strain deleted for both esterification genes and completely deficient in sterol esterification indicated that ARGP2 esterified cholesterol while ARGP1 did not. In contrast to ACAT1 and similar to liver esterification, the activity of ARGP2 was relatively resistant to a histidine active site modifier. ARGP2 is therefore a tissue-specific sterol esterification enzyme which we thus designated ACAT2. We speculate that ARGP1 participates in the coenzyme A-dependent acylation of substrate(s) other than cholesterol. Consistent with this hypothesis, ARGP1, unlike any other member of this multigene family, possesses a predicted diacylglycerol binding motif suggesting that it may perform the last acylation in triglyceride biosynthesis.
The chemical reactions and pathways involving diacylglycerol, a glycerol molecule substituted on the 1 and 2 hydroxyl groups with long chain fatty acyl residues. DAG is a normal intermediate in the biosynthesis of phosphatidyl phospholipids and is released from them by phospholipase C activity. DAG from phosphatidyl inositol polyphosphates is important in signal transduction.
The accumulation and maintenance in cells or tissues of lipids, compounds soluble in organic solvents but insoluble or sparingly soluble in aqueous solvents. Lipid reserves can be accumulated during early developmental stages for mobilization and utilization at later stages of development.
The chemical reactions and pathways involving long-chain fatty-acyl-CoAs, any derivative of coenzyme A in which the sulfhydryl group is in a thioester linkage with a long-chain fatty-acyl group. Long-chain fatty-acyl-CoAs have chain lengths of C13 or more.
AGPAT6 is a member of the 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) family that appears to be important in triglyceride biosynthesis in several tissues, but the precise biochemical function of the enzyme is unknown. In the current study, we show that AGPAT6 is a microsomal glycerol-3-phosphate acyltransferase (GPAT). Membranes from HEK293 cells overexpressing human AGPAT6 had higher levels of GPAT activity. Substrate specificity studies suggested that AGPAT6 was active against both saturated and unsaturated long-chain fatty acyl-CoAs. Both glycerol 3-phosphate and fatty acyl-CoA increased the GPAT activity, and the activity was sensitive to N-ethylmaleimide, a sulfhydryl-modifying reagent. Purified AGPAT6 protein possessed GPAT activity but not AGPAT activity. Using [(13)C(7)]oleic acid labeling and mass spectrometry, we found that overexpression of AGPAT6 increased both lysophosphatidic acid and phosphatidic acid levels in cells. In these studies, total triglyceride and phosphatidylcholine levels were not significantly altered, although there were significant changes in the abundance of specific phosphatidylcholine species. Human AGPAT6 is localized to endoplasmic reticulum and is broadly distributed in tissues. Membranes of mammary epithelial cells from Agpat6-deficient mice exhibited markedly reduced GPAT activity compared with membranes from wild-type mice. Reducing AGPAT6 expression in HEK293 cells through small interfering RNA knockdown suggested that AGPAT6 significantly contributed to HEK293 cellular GPAT activity. Our data indicate that AGPAT6 is a microsomal GPAT, and we propose renaming this enzyme GPAT4.
The chemical reactions and pathways involving triglyceride, any triester of glycerol. The three fatty acid residues may all be the same or differ in any permutation. Triglycerides are important components of plant oils, animal fats and animal plasma lipoproteins.
J. Biol. Chem. 273, 26765-26771 (1998)[PubMed:9756920]
The enzyme acyl coenzyme A:cholesterol acyltransferase 1 (ACAT1) mediates sterol esterification, a crucial component of intracellular lipid homeostasis. Two enzymes catalyze this activity in Saccharomyces cerevisiae (yeast), and several lines of evidence suggest multigene families may also exist in mammals. Using the human ACAT1 sequence to screen data bases of expressed sequence tags, we identified two novel and distinct partial human cDNAs. Full-length cDNA clones for these ACAT related gene products (ARGP) 1 and 2 were isolated from a hepatocyte (HepG2) cDNA library. ARGP1 was expressed in numerous human adult tissues and tissue culture cell lines, whereas expression of ARGP2 was more restricted. In vitro microsomal assays in a yeast strain deleted for both esterification genes and completely deficient in sterol esterification indicated that ARGP2 esterified cholesterol while ARGP1 did not. In contrast to ACAT1 and similar to liver esterification, the activity of ARGP2 was relatively resistant to a histidine active site modifier. ARGP2 is therefore a tissue-specific sterol esterification enzyme which we thus designated ACAT2. We speculate that ARGP1 participates in the coenzyme A-dependent acylation of substrate(s) other than cholesterol. Consistent with this hypothesis, ARGP1, unlike any other member of this multigene family, possesses a predicted diacylglycerol binding motif suggesting that it may perform the last acylation in triglyceride biosynthesis.
The relative importance of each core lipid in the assembly and secretion of very low density lipoproteins (VLDL) has been of interest over the past decade. The isolation of genes encoding diacylglycerol acyltransferase (DGAT) and acyl-CoA:cholesterol acyltransferases (ACAT1 and ACAT2) provided the opportunity to investigate the effects of isolated increases in triglycerides (TG) or cholesteryl esters (CE) on apolipoprotein B (apoB) lipoprotein biogenesis. Overexpression of human DGAT1 in rat hepatoma McA-RH7777 cells resulted in increased synthesis, cellular accumulation, and secretion of TG. These effects were associated with decreased intracellular degradation and increased secretion of newly synthesized apoB as VLDL. Similarly, overexpression of human ACAT1 or ACAT2 in McA-RH7777 cells resulted in increased synthesis, cellular accumulation, and secretion of CE. This led to decreased intracellular degradation and increased secretion of VLDL apoB. Overexpression of ACAT2 had a significantly greater impact upon assembly and secretion of VLDL from liver cells than did overexpression of ACAT1. The addition of oleic acid (OA) to media resulted in a further increase in VLDL secretion from cells expressing DGAT1, ACAT1, or ACAT2. VLDL secreted from DGAT1-expressing cells incubated in OA had a higher TG:CE ratio than VLDL secreted from ACAT1- and ACAT2-expressing cells treated with OA. These studies indicate that increasing DGAT1, ACAT1, or ACAT2 expression in McA-RH7777 cells stimulates the assembly and secretion of VLDL from liver cells and that the core composition of the secreted VLDL reflects the enzymatic activity that is elevated.
We provide biochemical evidence that enzymes involved in the synthesis of triacylglycerol, namely acyl coenzyme A:diacylglycerol acyltransferase (DGAT) and acyl coenzyme A:monoacylglycerol acyltransferase (MGAT), are capable of carrying out the acyl coenzyme A:retinol acyltransferase (ARAT) reaction. Among them, DGAT1 appears to have the highest specific activity. The apparent K(m) values of recombinant DGAT1/ARAT for retinol and palmitoyl coenzyme A were determined to be 25.9+/-2.1 microM and 13.9+/-0.3 microM, respectively, both of which are similar to the values previously determined for ARAT in native tissues. A novel selective DGAT1 inhibitor, XP620, inhibits recombinant DGAT1/ARAT at the retinol recognition site. In the differentiated Caco-2 cell membranes, XP620 inhibits approximately 85% of the Caco-2/ARAT activity indicating that DGAT1/ARAT may be the major source of ARAT activity in these cells. Of the two most abundant fatty acyl retinyl esters present in the intact differentiated Caco-2 cells, XP620 selectively inhibits retinyl-oleate formation without influencing the retinyl-palmitate formation. Using this inhibitor, we estimate that approximately 64% of total retinyl ester formation occurs via DGAT1/ARAT. These studies suggest that DGAT1/ARAT is the major enzyme involved in retinyl ester synthesis in Caco-2 cells.
We provide biochemical evidence that enzymes involved in the synthesis of triacylglycerol, namely acyl coenzyme A:diacylglycerol acyltransferase (DGAT) and acyl coenzyme A:monoacylglycerol acyltransferase (MGAT), are capable of carrying out the acyl coenzyme A:retinol acyltransferase (ARAT) reaction. Among them, DGAT1 appears to have the highest specific activity. The apparent K(m) values of recombinant DGAT1/ARAT for retinol and palmitoyl coenzyme A were determined to be 25.9+/-2.1 microM and 13.9+/-0.3 microM, respectively, both of which are similar to the values previously determined for ARAT in native tissues. A novel selective DGAT1 inhibitor, XP620, inhibits recombinant DGAT1/ARAT at the retinol recognition site. In the differentiated Caco-2 cell membranes, XP620 inhibits approximately 85% of the Caco-2/ARAT activity indicating that DGAT1/ARAT may be the major source of ARAT activity in these cells. Of the two most abundant fatty acyl retinyl esters present in the intact differentiated Caco-2 cells, XP620 selectively inhibits retinyl-oleate formation without influencing the retinyl-palmitate formation. Using this inhibitor, we estimate that approximately 64% of total retinyl ester formation occurs via DGAT1/ARAT. These studies suggest that DGAT1/ARAT is the major enzyme involved in retinyl ester synthesis in Caco-2 cells.
We provide biochemical evidence that enzymes involved in the synthesis of triacylglycerol, namely acyl coenzyme A:diacylglycerol acyltransferase (DGAT) and acyl coenzyme A:monoacylglycerol acyltransferase (MGAT), are capable of carrying out the acyl coenzyme A:retinol acyltransferase (ARAT) reaction. Among them, DGAT1 appears to have the highest specific activity. The apparent K(m) values of recombinant DGAT1/ARAT for retinol and palmitoyl coenzyme A were determined to be 25.9+/-2.1 microM and 13.9+/-0.3 microM, respectively, both of which are similar to the values previously determined for ARAT in native tissues. A novel selective DGAT1 inhibitor, XP620, inhibits recombinant DGAT1/ARAT at the retinol recognition site. In the differentiated Caco-2 cell membranes, XP620 inhibits approximately 85% of the Caco-2/ARAT activity indicating that DGAT1/ARAT may be the major source of ARAT activity in these cells. Of the two most abundant fatty acyl retinyl esters present in the intact differentiated Caco-2 cells, XP620 selectively inhibits retinyl-oleate formation without influencing the retinyl-palmitate formation. Using this inhibitor, we estimate that approximately 64% of total retinyl ester formation occurs via DGAT1/ARAT. These studies suggest that DGAT1/ARAT is the major enzyme involved in retinyl ester synthesis in Caco-2 cells.
A reference proteome is a set of protein sequences derived from a complete proteome which constitutes a defined standard for a particular user community. Reference proteomes are manually defined according to a number of criteria. They cover the proteomes of well- studied model organisms and other proteomes of interest for biomedical and biotechnological research. Reference proteomes have been selected to provide broad coverage of the tree of life, and constitute a representative cross-section of the taxonomic diversity to be found within UniProtKB.
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