Hepatic Monoacylglycerol Acyltransferase Research Articles

Hepatic monoacylglycerol acyltransferase 1 is induced by prolonged food deprivation to modulate the hepatic fasting response

During prolonged fasting, the liver plays a central role in maintaining systemic energy homeostasis by producing glucose and ketones in processes fueled by oxidation of fatty acids liberated from adipose tissue. In mice, this is accompanied by transient hepatic accumulation of glycerolipids. We found that the hepatic expression of monoacylglycerol acyltransferase 1 (Mogat1), an enzyme with monoacylglycerol acyltransferase (MGAT) activity that produces diacyl-glycerol from monoacylglycerol, was significantly increased in the liver of fasted mice compared with mice given ad libitum access to food. Basal and fasting-induced expression of Mogat1 was markedly diminished in the liver of mice lacking the transcription factor PPARα. Suppressing Mogat1 expression in liver and adipose tissue with antisense oligonucleotides (ASOs) reduced hepatic MGAT activity and triglyceride content compared with fasted controls. Surprisingly, the expression of many other PPARα target genes and PPARα activity was also decreased in mice given Mogat1 ASOs. When mice treated with control or Mogat1 ASOs were gavaged with the PPARα ligand, WY-14643, and then fasted for 18 h, WY-14643 administration reversed the effects of Mogat1 ASOs on PPARα target gene expression and liver triglyceride content. In conclusion, Mogat1 is a fasting-induced PPARα target gene that may feed forward to regulate liver PPARα activity during food deprivation.

Epigenetic programming at the Mogat1 locus may link neonatal overnutrition with long-term hepatic steatosis and insulin resistance.

Postnatal overfeeding increases the risk of chronic diseases later in life, including obesity, insulin resistance, hepatic steatosis, and type 2 diabetes. Epigenetic mechanisms might underlie the long-lasting effects associated with early nutrition. Here we aimed to explore the molecular pathways involved in early development of insulin resistance and hepatic steatosis, and we examined the potential contribution of DNA methylation and histone modifications to long-term programming of metabolic disease. We used a well-characterized mouse model of neonatal overfeeding and early adiposity by litter size reduction. Neonatal overfeeding led to hepatic insulin resistance very early in life that persisted throughout adulthood despite normalizing food intake. Up-regulation of monoacylglycerol O-acyltransferase ( Mogat) 1 conceivably mediates hepatic steatosis and insulin resistance through increasing intracellular diacylglycerol content. Early and sustained deregulation of Mogat1 was associated with a combination of histone modifications that might favor Mogat1 expression. In sum, postnatal overfeeding causes extremely rapid derangements of hepatic insulin sensitivity that remain relatively stable until adulthood. Epigenetic mechanisms, particularly histone modifications, could contribute to such long-lasting effects. Our data suggest that targeting hepatic monoacylglycerol acyltransferase activity during early life might provide a novel strategy to improve hepatic insulin sensitivity and prevent late-onset insulin resistance and fatty liver disease.-Ramon-Krauel, M., Pentinat, T., Bloks, V. W., Cebrià, J., Ribo, S., Pérez-Wienese, R., Vilà, M., Palacios-Marin, I., Fernández-Pérez, A., Vallejo, M., Téllez, N., Rodríguez, M. À., Yanes, O., Lerin, C., Díaz, R., Plosch, T., Tietge, U. J. F., Jimenez-Chillaron, J. C. Epigenetic programming at the Mogat1 locus may link neonatal overnutrition with long-term hepatic steatosis and insulin resistance.

Diradylglycerols alter fatty acid inhibition of monoacylglycerol acyltransferase activity in Triton X-100 mixed micelles.

The activity of hepatic monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22), a developmentally expressed microsomal enzyme, is inhibited by long-chain fatty acids, and stimulated by its product 1, 2-diacyl-sn-glycerol. Because the quantities of fatty acids and diacylglycerols are likely to vary in membranes during different physiological conditions and could thereby alter MGAT activity, we examined their combined effects on MGAT in Triton X-100/phospholipid mixed micelles. MGAT's product, 1,2-diC18:1-sn-glycerol, which is also normally a cooperative activator of the activity, reversed the 50% inhibition caused by 10 mol % oleic acid. The presence of oleic acid also allowed low concentrations (<10 mol %) of 1, 2-diC18:1-sn-glycerol to stimulate MGAT activity without the lag that is observed in the absence of fatty acid. At 12.6 mol %, 1, 2-monoC18:1-sn-glycerol ether, which alone has no effect on MGAT activity, became an activator in the presence of 10 mol % oleic acid. Kinetic studies revealed that in the presence of 15 mol % oleic acid, 1,2-diC18:1-sn-glycerol ether increased the apparent Vmax by 3. 8-fold while minimally altering the apparent Km for palmitoyl-CoA. Other neutral lipids including tri-C18:1-glycerol, ceramide, and cholesterol oleate did not stimulate MGAT in either the presence or the absence of fatty acid. Assay conditions altered MGAT's apparent relative preferences for potential monoradylglycerol substrates. The presence of phospholipids and of MGAT's 1,2-diacyl-sn-glycerol product increased the enzyme's apparent preference for its 2-monoacyl-sn-glycerol substrate by selectively increasing the apparent Vmax 2.7-fold only when 2-monoC18:1-sn-glycerol was the substrate. Thus, in addition to previously reported regulation of MGAT by phospholipids and intracellular lipid second messengers, these studies lend additional support to the hypothesis that changes in other membrane-associated lipids, such as long-chain fatty acids and diradylglycerols, may also profoundly alter the activity of MGAT.

Fatty acids and anionic phospholipids alter the palmitoyl coenzyme A kinetics of hepatic monoacylglycerol acyltransferase in Triton X-100 mixed micelles.

In order to gain a better understanding of the kinetics of activation and inhibition of hepatic monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22) by fatty acid, we examined the effect of fatty acid with respect to MGAT's long-chain acyl-CoA substrate in Triton X-100 mixed micelles. At concentrations between 2.5 and 5.3 mol %, oleic acid stimulated MGAT activity 2-fold, whereas oleic acid inhibited MGAT at concentrations higher than 7.5 mol %. The dependence on palmitoyl-CoA was highly cooperative with a Hill constant of greater than 2.4. When present at less than 3 mol%, oleic acid eliminated the lag in the dependence curve. When concentrations of oleic acid were higher than 3 mol %, Michaelis-Menton kinetics were observed with an apparent k(m) value of about 54 microM for palmitoyl-CoA but with progressively decreasing Vmax values. This effect was not observed with octanoic acid, suggesting that the medium-chain fatty acid is unable to associate stably with the mixed micelle and, thus, cannot substantially alter substrate affinity. When anionic phospholipids were tested, phosphatidic acid, lysophosphatidic acid, phosphatidylserine, and phosphatidylinositol eliminated some of the lag in activation by palmitoyl-CoA. At high molar concentrations of the anionic lipid activators, apparent k(m) values ranged from 77 microM for phosphatidic acid to 196 microM for phosphatidylinositol. Zwitterionic phospholipids had no effect, nor did the non-phospholipid activators bovine serum albumin or sn-1,2-diacylglycerol. CaCl2, but not neomycin or KC1, could overcome the inhibitory effect of oleic acid; thus, the inhibitory effect of fatty acid did not appear to occur by electrostatic interactions. These blockers did not change the effects observed with the anionic phospholipid activators or with the inhibitor, sphingosine. An altered k(m) for palmitoyl-CoA in the presence of fatty acid or anionic phospholipid suggests that both long-chain fatty acids and phospholipid cofactors may induce a conformational change in MGAT, thereby altering the enzyme's affinity for its long-chain acyl-CoA substrate. These data further support the hypothesis that the synthesis of glycerolipids via the monoacylglycerol pathway may be highly regulated via a variety of lipid second messengers such as phosphatidic acid and diacylglycerol, as well as by the influx of fatty acids derived from high-fat diets, or from the hydrolysis of adipocyte triacylglycerol during fasting or diabetes.

Sphingosine Inhibits Rat Hepatic Monoacylglycerol Acyltransferase in Triton X-100 Mixed Micelles and Isolated Hepatocytes

Hepatic monoacylglycerol acyltransferase (MGAT), a developmentally-regulated microsomal activity that catalyzes the synthesis of sn-1,2-diacylglycerol, is regulated by anionic phospholipids and sn-1,2-diacylglycerol in Triton X-100 mixed micelles. Spingomyelin stimulated MGAT activity, whereas sphingosine, sphinganine, phytosphingosine, and stearylamine were inhibitors (IC50 of 9, 5.5, 5, and 6 mol %, respectively). Since ceramide and octylamine had relatively little effect, inhibition appears to require a free amino group and a long-chain hydrocarbon. Inhibition by sphingosine was competitive with respect to phosphatidic acid, phosphatidylinositol, or phosphatidylserine, suggesting that anionic phospholipids may activate MGAT at a specific site that is competitively blocked by sphingolipids. Both sphingosine and sphinganine inhibited MGAT activity in cultured hepatocytes from 10-day-old rats in a dose-dependent manner. Stimulation of MGAT activity by diacylglycerol was specific for sn-1,2-stereoisomers that contained two long fatty acyl chains. The diacylglycerol analogs phorbol 12-myristyl 13-acetate and ceramide had no effect. The highly cooperative activation of MGAT by sn-1,2-diacylglycerol was also inhibited by sphingosine. It is unlikely that activation of MGAT by low molar concentrations of anionic phospholipids is solely due to electrostatic interactions between the enzyme and negatively charged lipids because high ionic strength, neomycin, and Ca2+ had similar effects on enzyme activity irrespective of the presence or absence of phosphatidic acid. These data suggest that MGAT activity may be regulated physiologically by specific intermediates of glycerolipid metabolism and that, in neonatal rat liver, signal transduction may be linked to the synthesis of complex lipids via the monoacylglycerol pathway.

Hepatic monoacylglycerol acyltransferase is regulated by sn-1,2-diacylglycerol and by specific lipids in Triton X-100/phospholipid-mixed micelles.

The lipid cofactor requirement of hepatic monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22) was studied in Triton X-100/lipid-mixed micelles. Anionic phospholipids and anionic lysophospholipids stimulated MGAT activity, whereas fatty acids and sphingosine inhibited enzyme activity. Phosphatidic acid was a potent activator, stimulating MGAT 11-fold at 4.2 mol %. Kinetic studies revealed that phosphatidic acid, with an apparent Ka of 0.26 mol %, was a better activator than phosphatidylserine, phosphatidylinositol, or cardiolipin. Of the anionic lysophospholipids, lysophosphatidic acid was a better activator than lysophosphatidylserine, stimulating maximally at less than 3 mol %. Oleate was a more potent inhibitor (Ki, 2.4 mol %) than sphingosine (Ki, 18.3 mol %). The dependence of MGAT on sn-2-monoacylglycerol was not cooperative in the absence or presence of anionic phospholipids, oleic acid, or sphingosine. The apparent Km for sn-2-monoC18:1-glycerol was 1.24 mol % in the presence of maximally activating phospholipid and 0.19 mol % when phospholipid was omitted. MGAT's product sn-1,2-diacylglycerol was a weaker activator than the anionic phospholipids, but the effects of diacylglycerol and phospholipid were additive. Activation by sn-1,2-diC18:1-glycerol was highly cooperative with a Hill coefficient of 3.6. Activation was specific for the sn-1,2-stereoisomer; neither 1,3-diacylglycerol nor the ether analogs of sn-1,2- or 1,3-diacylglycerol were activators. Since several of the lipid modulators of MGAT activity are intracellular second messengers, these data suggest the possibility that regulatory links exist between signal transduction and the synthesis of complex lipids via the monoacylglycerol pathway.

Hepatic monoacylglycerol acyltransferase is regulated by sn-1, 2-diacylglycerol and by specific lipids in Triton X-100/phospholipid-mixed micelles.

Hepatic monoacylglycerol acyltransferase is regulated by sn-1, 2-diacylglycerol and by specific lipids in Triton X-100/phospholipid-mixed micelles.

Selective retention of essential fatty acids: the role of hepatic monoacylglycerol acyltransferase.

In the suckling rat, chick embryo, and hibernating marmot, fatty acids provide the major source of energy, and despite the high rate of hepatic beta-oxidation, these animals selectively retain long-chain polyunsaturated derivatives of C18:2n-6 and C18:3n-3. To determine whether the hepatic microsomal activity monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22) could provide a mechanism to selectively acylate monoacylglycerols that contain essential fatty acids, we tested the ability of MGAT activity from each of the three species to acylate sn-2-monoC18:1-, sn-2-monoC18:2-, sn-2-monoC18:3-, and sn-2-monoC20:4-glycerols. Hepatic MGAT activity acylated sn-2-monoC18:3-glycerol and sn-2-monoC18:2-glycerol in preference to sn-2-monoC18:1-glycerol in each of the three different lipolytic animals. MGAT's acyl group specificity could not be explained by invoking differences in membrane fluidity because the apparent affinity for sn-2-monoC20:4-glycerol was not increased. Further, sn-2-monoC18:3-glycerol remained a preferred substrate under assay conditions when both the C18:3 and C18:1 species were present in equal amounts. As would be predicted in the presence of high activity of a selective MGAT, the hepatic glycerolipids from neonatal rats showed increases in dienoic, trienoic, and C22:6 fatty acids and relative decreases in monoenoic, saturated, and C20:4 fatty acids. We hypothesize that, during lipolysis, the reacylation of sn-2-monoacylglycerols by MGAT may provide a mechanism by which essential fatty acids are retained within specific tissues.

Increased hepatic monoacylglycerol acyltransferase activity in streptozotocin-induced diabetes: Characterization and comparison with activities from adult and neonatal rat liver

Hepatic monoacylglycerol acyltransferase (EC 2.3.1.22) (MGAT) is a developmentally-expressed activity associated with physiological periods characterized by high rates of lipolysis and dependence on fatty acids for energy production. During these periods, MGAT may help to retain essential fatty acids selectively. In streptozotocin-diabetes, mean MGAT-specific activity increased 11.8-fold. We characterized microsomal MGAT activity from diabetic and control adult rats, and compared these adult activities with the high neonatal activity. Compared with the activity in neonatal liver, adult MGAT activity was more thermolabile, had a markedly different pH profile, and responded differently to incubation with bovine serum albumin, phospholipids, and MnCl 2. Adult diabetic MGAT activity was also stimulated 2-fold by albumin and was markedly thermolabile, but was not inhibited by phospholipid. Diabetic MGAT activity had some properties that combined characteristics observed in adult and neonatal microsomes: a pH dependence that was optimal at pH 7.0 but that plateaued between pH 7.0 and 9.5, and neither stimulation nor inhibition after incubation with MnCl 2. Diabetic MGAT acylated monoalkylglycerols more readily than did either the neonatal or the adult MGAT activities. The enhanced expression of hepatic MGAT activity in diabetes is consistent with its postulated role in retaining essential fatty acids during lipolysis.

Hepatic monoacylglycerol acyltransferase activity in HA1 and HA7 hepatoma/hepatocyte hybrid cells: Regulation by insulin and dexamethasone and by cell density

Hepatic monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22) is a developmentally-cxpressed enzyme that catalyzes the stereospecific synthesis of sn-1,2-diacylglycerol from sn-2-monoacylglycerol and long-chain fatty acyl-CoA. In order to study the regulation of MGAT, we developed a rapid assay that can be performed directly on permeabilized HA rat hepatocyte/hepatoma hybrid cells, a line that expresses levels of hepatic MGAT activity and a lipogenic program characteristic of fetal hepatocytes. In permeabilized HA cells, MGAT activity was proportional to the time of incubation and was highly dependent on added sn-2-monoacylglycerol and palmitoyl-CoA. The apparent K m values were 16.6 and 12.7 μM for palmitoyl-CoA and 2-mono-oleoylglycerol, respectively. Activity was low with the 1(3)- and sn-2-ether analogs of monooleoylglycerol, supporting the conclusion that the cells express the hepatic isoenzyme of MGAT. MGAT activity increased directly with cell density and was unrelated to the number of days in culture. Long-term incubation (2–4 days) of HA cells with various hormones (including triiodothyronine, human placental lactogen, epidermal growth factor, glucagon and growth hormone) showed that only a combination of dexamethasome and insulin resulted in significantly decreased MGAT activity. None of these hormones affected MGAT activity in short-term (0.5–4 h) incubations. These studies suggest that the developmental decline in rat hepatic MGAT activity may be regulated by glucocorticoids and insulin, hormones that increase during and after the second postnatal week.

Hepatic monoacylglycerol acyltransferase: ontogeny and characterization of an activity associated with the chick embryo.

Hepatic monoacylglycerol acyltransferase is expressed during the perinatal period in rats and guinea pigs and appears to be related temporally to the availability of fatty acids and to the development of hepatic steatosis. In order to determine when monoacylglycerol acyltransferase activity is expressed in an avian species, its ontogeny was investigated in chick liver total particulate preparations. In livers from 11- to 21-day-old chick embryos, monoacylglycerol acyltransferase specific activity was 34.5 +/- 8.1 nmol/min per mg of total particulate protein. The specific activity decreased 93% to 2.6 +/- 1.3 nmol/min per mg by the 6th day after hatching. The specific activities of fatty acid CoA ligase, diacylglycerol acyltransferase, and microsomal and mitochondrial glycerol-P acyltransferases changed comparatively little during this time period. In the embryos, the monoacylglycerol acyltransferase activity per liver rose 28-fold between the 11th and 21st day, corresponding exactly to the increase in liver total particulate protein during this time. Monoacylglycerol acyltransferase activity in other tissues was 25- to 115-fold lower than observed in liver. Optimal activity was measured using 25 microM palmitoyl-CoA and 50 microM sn-2-monooleoylglycerol. The activity with the 1- and 2-monooleoylglycerol ethers and 1-monooleoylglycerol was very low. In contrast to microsomes from rat liver, about 70% of the product with the 1- and 2-monooleoylglycerol ethers was triradylglycerol, suggesting that the diacylglycerol acyltransferase from chick liver can acylate acyl, alkylglycerols. The activity with sn-2-monooleoylglycerol amide was 12.5% of that observed with the corresponding 2-monooleoylglycerol suggesting that the ester bond is important; the 1-monooleoylglycerol amide was not a substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of microsomal activities of triacylglycerol synthesis in guinea pig liver.

Because the onset of triacylglycerol-rich lipoprotein synthesis occurs in guinea pig liver during fetal life, we investigated the microsomal enzyme activities of triacylglycerol synthesis in fetal and postnatal guinea pig liver. Hepatic monoacylglycerol acyltransferase specific and total microsomal activities peaked by the 50th day of gestation and declined rapidly after birth to levels that were virtually unmeasurable in the adult. Peak fetal specific activity was more than 75-fold higher than observed in the adult. The specific activities of fatty acid CoA ligase and lysophosphatidic acid acyltransferase increased 2- to 3-fold before birth; lysophosphatidic acid acyltransferase increased a further 2.6-fold during the first week of life. Specific activities of phosphatidic acid phosphatase, microsomal glycerophosphate acyltransferase, and diacylglycerol acyltransferase varied minimally over the time course investigated. These data demonstrate that selective changes occur in guinea pig hepatic microsomal activities of triacylglycerol synthesis before birth. Because of an approximate 11-fold increase in hepatic microsomal protein between birth and the adult, however, major increases in total microsomal activity of all the triacylglycerol synthetic activities occurred after birth. The pattern of monoacylglycerol acyltransferase specific and total microsomal activities differs from that of the rat in occurring primarily during the last third of gestation instead of during the suckling period. This pattern provides evidence that hepatic monoacylglycerol acyltransferase activity probably does not function to acylate 2-monoacylglycerols derived from partial hydrolysis of diet-derived triacylglycerol.

Monoacylglycerol acyltransferase. Evidence that the activities from rat intestine and suckling liver are tissue-specific isoenzymes.

The monoglycerol acyltransferase (EC 2.3.1.22) (recommended name acylglycerol palmitoltransferase) activities from rat intestinal mucosa and suckling liver microsomes were compared in order to determine why substrate specificities differed in the two tissues. Suckling liver monoacylglycerol acyltransferase activity was highly specific for sn-2-mono-C18:1 glycerol and acylated rac-1-mono-C18:1 glycerol and 1- and 2-mono-C18:1 glycerol ethers poorly. In contrast, the substrate specificity of intestinal monoacylglycerol acyltransferase activity was broad. 1-Acyl- and 1- and 2-alkylglycerols were acylated at rates that were 45-78% of the rate observed with the preferred substrate sn-2-mono-C18:1 glycerol. Partial heat inactivation did not alter these relative specific activities, making it unlikely that intestinal microsomes contained a second acyltransferase capable of acylating the alternate substrates. The hypothesis that intestine and liver contain non-identical monoacylglycerol acyltransferase activities was further tested. Intestinal mucosa monoacylglycerol acyltransferase was much more thermolabile than the liver activity. Incubation with 50 microM diethylpyrocarbonate inactivated liver monoacylglycerol acyltransferase activity 84% but had little effect on the intestinal activity. Hydroxylamine completely reversed diethylpyrocarbonate inactivation, suggesting that critical histidine residues were more accessible in liver monoacylglycerol acyltransferase. 2,4,6-Trinitrobenzene sulfonic acid inactivated hepatic monoacylglycerol acyltransferase more than the intestinal activity, suggesting that critical lysine residues were more accessible. The intestinal and liver activities were also differently affected by acetone, detergents, MgCl2, phospholipids, and bovine serum albumin. Taken as a whole, the data strongly suggest that rat intestinal mucosa and suckling liver contain tissue-specific monoacylglycerol acyltransferase isoenzymes.

Subcellular location and topography of rat hepatic monoacylglycerol acyltransferase activity

Monoacylglycerol acyltransferase activity from suckling rat liver was localized to the microsomal subcellular fraction by differential centrifugation and comparison with the partitioning of selected marker enzymes. Chymotrypsin, pronase, and proteinase K inactivated the monoacylglycerol acyltransferase activity in detergent-disrupted microsomes, but not in intact microsomes, falsely suggesting a lumenal location for the enzyme. The impermeant inhibitors mercury-dextran and 4,4'-diisothiocyano,-2.2'-disulfonic acid stilbene inhibited monoacylglycerol acyltransferase in intact microsomes. These data, as well as the lack of latency and the inability of the substrate palmitoyl-CoA to readily permeate hepatic microsomes from suckling rats, strongly suggest that the enzyme's active site faces the cytosolic surface of the endoplasmic reticulum.

Hepatic monoacylglycerol acyltransferase. Characterization of an activity associated with the suckling period in rats.

Hepatic monoacylglycerol acyltransferase activity is 700-fold higher during the suckling period than in the adult rat. Specific activity in total particulate preparations rose from 9.4 nmol/min/mg before birth to a peak of 78 nmol/min/mg on the 6th to 8th postnatal days. Monoacylglycerol acyltransferase activity fell sharply after day 8 and was 1.6 and 0.1 nmol/min/mg on day 28 and in adult rats, respectively. The activity had a pH optimum at 8.0 and was activated by albumin and by phospholipids. With [3H]palmitoyl-CoA and sn-2-monooleoylglycerol, more than 96% of the products were di- and triacylglycerols. More than 92% of the diacylglycerol product was the 1,2 isomer. The activity was stable at 43 degrees C for 50 min. Thermal inactivation showed t 1/2 values of 8 min and 4.5 min at 53.5 and 55 degrees C, respectively. In suckling rats, monoacylglycerol acyltransferase activities in liver and intestinal mucosa were 150- to 800-fold higher than in other tissues. Monoacylglycerol acyltransferase activity was 12.5-fold greater with palmitoyl-CoA than with octanoyl-CoA. Acetyl-CoA was not a substrate. The sn-2-monoacylglycerols were strongly preferred over the sn-1 isomers. No direct relationship was noted between 2-monoacylglycerol chain length and apparent Km value. The presence of high levels of monoacylglycerol acyltransferase activity in suckling rat liver suggests that the monoacylglycerol pathway functions as a major route of hepatic glycerolipid synthesis during the suckling period but not in adult animals.

HEPATIC MONOACYLGLYCEROL ACYLTRANSFERASE ACTIVITY IN SUCKLING RATS

Hepatic monoacylglycerol acyltransferase (MGAT) activity is 700-fold higher during the suckling period than in the adult rat. Specific activity in total particulate preparations rose from 9.4 nmol/min/mg before birth to a peak of 78 nmol/min/mg on the 6th postnatal day. MGAT activity fell sharply after day 8 and was 1.6 and 0.1 nmol/min/mg on day 28 and in adult rats, respectively. The activity had a pH optimum at 8.0 and was activated by albumin and by phospholipids. With (3H)palmitoyl-CoA and sn-2-monooleoylglycerol, more than 96% of the products were di- and triacylglycerols. 90% of the diacylglycerol product was the 1, 2 isomer. The activity was stable at 43° for 50 min and had t 1/2 values of 8 min and 4.5 min at 53.5 and 55°C, respectively. Suckling rat MGAT activities in liver and intestinal mucosa were 100- to 300-fold higher than in other tissues. MGAT activity was 12.5 fold greater with C16:0-CoA than with C8:0-CoA. Acetyl-CoA was not a substrate. The sn-2-monoacylglycerols were strongly preferred over the sn-1 isomers. No direct relationship was noted between monoacylglycerol chain length and apparent Km value. The presence of high MGAT activity suggests that the monoacylglycerol pathway provides a major route of glycerolipid synthesis in suckling rat liver.