This is almost certainly a genetic condition, with an autosomal recessive pattern of inheritance.

If isolated adipocytes are incubated with fatty acids, they only synthesise triacylglycerol if they are also provided with glucose.

The most likely source of glycerol will be glycerol 3-phosphate, which arises as a result of the reduction of dihydroxyacetone phosphate, an intermediate in glycolysis.

Remember that fatty acids are never in free solution in cells; they are always esterified to either CoA or carnitine. Fatty acyl carnitine is formed only to transport fatty acids into the mitochondrion for beta-oxidation (see the exercise on Muscle weakness, heart failure and profound hypoglycaemia in a young girl).

This means that fatty acids are activated for triacylglycerol synthesis by esterification with CoA.

They would lyse membranes - soap is a mixture of sodium salts of fatty acids. They would also form insoluble precipitates with calcium ions.

If non-esterified fatty acids were in free solution in the bloodstream they would lyse red cell membranes and risk blocking blood vessels with precipitated calcium salts (as well as causing severe disturbance to plasma calcium ion concentrations.

Non-esterified fatty acids are transported bound to serum albumin, which has a high affinity binding site for fatty acids.

One of the products of the reaction is pyrophosphate (PPi). Pyrophosphate is rapidly hydrolysed by pyrophosphatase, yielding two mol of inorganic phosphate. This means that one of the products of the reaction is unavailable to undergo the back reaction.

In order to investigate TGL's condition, biopsies of her (almost negligible) subcutaneous adipose tissue and liver were take, and were incubated with [14C]palmitate (C16:0) and [14C]linoleate (C18:2 n-6), together with glucose and insulin. Similar incubations were set up using adipose tissue and liver biopsy samples from a control subject. Esterified lipids were separated by high pressure liquid chromatography, and the radioactivity in each fraction was measured. The results were as follows (in dpm per mg DNA in the tissue samples ± sd for three replicate incubations):

Looking at the results for the control subject, it is obvious that there is stepwise addition of fatty acids to glycerol 3-phosphate, forming monoacylglycerol phosphate, then diacylglycerol phosphate, and finally triacylglycerol.

You might also deduce that phospholipids are synthesised from diacylglycerol phosphate, and that this is dephosphorylated to diacylglycerol before esterification of the third fatty acid.

The pathway of TAG synthesis is shown on the right.

TGL's hepatocytes seem to be able to synthesise triacylglycerol more or less normally, but her adipocytes can only synthesise monoacylglycerol phosphate, not diacylglycerol phosphate, phospholipids or triacylglycerol.

This suggests that while she has normal activity of glycerol 3-phosphate acyltransferase, which catalyses esterification of glyceraldehyde 3-phosphate to monoacylglycerol phosphate , in adipose tissue (but not in liver) she lacks the next enzyme, lysophosphatidic acid acyltransferase, which catalyses esterification of monoacylglycerol phosphate to diacylglycerol phosphate

This suggests that there are two isoenzymes of lysophosphatidic acid acyltransferase, one in the liver and a different one in adipose tissue; TGL has little or no activity of the adipose tissue enzyme, but adequate activity of the hepatic enzyme.

In the next set of experiments isolated rat hepatocytes were incubated with palmitate (C16:0) and linoleate (C18:2 n-6); in one set of experiments [14C]palmitate and unlabelled linoleate was used and in the other unlabelled palmitate and [14C]linoleate were used. The results were as follows (in dpm per mg DNA in the hepatocytes for three replicate incubations):

Label from palmitate is incorporated at carbon-1 and carbon-3 of glycerol, whereas that from linoleate is incorporated at carbon-2. This suggests that lysophosphatidic acid 2-acyltransferase has a higher affinity for, or activity with, polyunsaturated fatty acids than saturated. By contrast, glycerol 3-phosphate 1-acyltransferase and diacylglycerol 3-acyltransferase have higher affinity for, or activity towards, saturated than polyunsaturated fatty acids.

The lack of adipose tissue triacylglycerol is obviously attributable to her inability to synthesise diacylglycerol phosphate in adipose tissue. In addition to synthesis of fatty acids from glucose, adipose tissue takes up fatty acids from plasma lipoproteins as a result of extra-cellular hydrolysis; it cannot take up preformed triacylglycerol.

In the fed state much of the triacylglycerol in chylomicrons is hydrolysed as they pass through adipose tissue, as a result of the activity of extracellular lipoprotein lipase. Most of the fatty acids released are taken up by adipose tissue and re-esterified to form triacylglycerol. This is not possible in TGL, so the fatty acids released by lipoprotein lipase will remain in the circulation, and be taken up by the liver for esterification. (Remember that her liver lysophosphatidic acid acyltransferase is unimpaired.)

Triacylglycerol synthesised in the liver from fatty acids released into the circulation by lipoprotein lipase, as well as that in chylomicron remnants and newly synthesised fatty acids, is released into the circulation in very low density lipoprotein. Again this will be subject to lipoprotein lipase action in adipose tissue, but the adipose tissue will not be able to take up and use the fatty acids, which will again be released into the circulation and taken up by the liver, for re-esterification and release in very low density lipoprotein.

Almost certainly this is fatty infiltration of the liver (inappropriate storage of triacylglycerol in vacuoles in the liver, steatosis) as a result of the inability of her adipose tissue to take up fatty acids from triacylglycerol in plasma lipoproteins.

This would almost certainly be lethal early in fetal development. Quite apart from the synthesis of triacylglycerol, the liver enzyme is essential for the synthesis of phospholipids that are required for cell membrane synthesis.

This is partly due to the lack of adipose tissue over her muscles and partly due to increased uptake of fatty acids into muscle reserves of triacylglycerol, because of the high circulating concentration of triacylglycerol in chylomicrons and very low density lipoprotein.

The simplest, and possibly the only, way would be to limit her fat intake quite severely, perhaps to 20 - 25% of energy from fat (compared with an average of 35 - 40%, and a desirable intake of 30% of energy form fat). This will minimise the amount of triacylglycerol in chylomicrons and hence the amount of triacylglycerol that will be re-exported from the liver in very low density lipoprotein.

Extreme emaciation in patient with advanced cancer

Many patients with advanced cancer show extreme emaciation and wasting, with a higher than normal basal metabolic rate (hypermetabolism). Part of this is due to increased cycling between anaerobic glycolysis in the tumour and gluconeogenesis in the liver (see the exercise on Weight loss in a patient with advanced cancer).

However, there is also increased cycling of lipids, with activation of hormone sensitive lipase in adipose tissue by a proteoglycan secreted by the tumour, leading to release of fatty acids into the circulation that are taken up by the liver and re-esterified for export in very low density lipoprotein.

We saw above that each mol of fatty acid esterified to CoA requires the hydrolysis of ATP to AMP and pyrophosphate, and hence is equivalent to 2 x ATP per mol of fatty acid esterified, or 6 x ATP per mol of triacylglycerol synthesised.

There is therefore a requirement for increased oxidation of metabolic fuels to provide this ATP; unless the patient can be persuaded to eat more food (or be provided with intravenous nutritional support), this increased oxidative metabolism will lead to a considerable loss of adipose tissue triacylglycerol reserves, and hence wasting and emaciation.

Key points from this exercise:

• The glycerol moiety of triacylglycerol arises from glycerol 3-phosphate, which is formed by reduction of the glycolytic intermediate dihydroxyacetone phosphate.
• Fatty acids are bound to serum albumin in the bloodstream, and esterified with either carnitine or CoA in cells because free fatty acids will lyse cell membranes and precipitate insoluble calcium salts.
• Fatty acids are activated for triacylglycerol synthesis by esterification with CoA. There is a cost equivalent to 2 x ATP for each mol of fatty acid esterified to form acyl CoA, because the reaction involves the hydrolysis of ATP to AMP and pyrophosphate.
• The sequence of reactions in triacylglycerol synthesis is as follows:
• A fatty acid from acyl CoA is esterified to carbon-1 of glycerol phosphate, forming 1-monoacylglycerol 3-phosphate (also known as lysophosphatidic acid). This is commonly a saturated fatty acid because glycerol 3-phosphate 1-acyltransferase has a higher affinity for, or activity towards, saturated rather than unsaturated fatty acids.
• A fatty acid from acyl CoA is esterified to carbon-2 of 1-monoacylglycerol 3-phosphate, forming diacylglycerol 3-phosphate (also known as phosphatidic acid). This is commonly a polyunsaturated fatty acid. This is because lysophosphatidic acid 2-acyltransferase has a higher affinity for, or activity towards, polyunsaturated rather than saturated fatty acids.
• Diacylglycerol 3-phosphate is hydrolysed to diacylglycerol, then a third fatty acid from fatty acyl CoA is esterified to carbon-3 of the glycerol, forming a triacylglycerol. This is commonly a saturated fatty acid. This is because diacylglycerol 3-acyltransferase has a higher affinity for, or activity towards, saturated rather than unsaturated fatty acids.
• Diacylglycerol 3-phosphate (phosphatidic acid) is also the precursor of membrane and other phospholipids.
• The enzyme that catalyses the esterification of 1-monoacylglycerol 3-phosphate, forming diacylglycerol 3-phosphate (lysophosphatidic acid 2-acyltransferase) is inactive in the adipose tissue of patients with congenital generalised lipodystrophy, but the liver enzyme is unaffected. Deficiency of the liver enzyme would almost certainly be lethal early in fetal development because of the severe impairment of phospholipid synthesis.
• Patients with congenital generalised lipodystrophy have negligible reserves of subcutaneous and abdominal adipose tissue because adipose tissue is unable to esterify fatty acids to form triacylglycerol. They have hypertriglyceridaemia because triacylglycerol in chylomicrons and that exported from the liver in very low density lipoprotein cannot be used by adipose tissue for triacylglycerol synthesis, and fatty acids arising from lipoprotein lipase activity in the circulation are returned to the liver for re-esterification and re-export in very low density lipoprotein. There is also accumulation of triacylglycerol in droplets in the liver - hepatic steatosis. Patients appear very muscular partly because of the lack of adipose tissue over their muscles and partly due to increased uptake of fatty acids into muscle reserves of triacylglycerol, because of the high circulating concentration of triacylglycerol in chylomicrons and very low density lipoprotein.
• In advanced cancer patients are hypermetabolic partly because of activation of hormone sensitive lipase in adipose tissue by a proteoglycan secreted by the tumour; this leads to release of non-esterified fatty acids that are re-esterified in the liver, at a cost of 2 x ATP per fatty acid (or 6 x ATP per mol of triacylglycerol).