Liver triacylglycerol biosynthesis

Thyroid hormones stimulate the rate of triacylglycerol biosynthesis in liver but have the opposite effect in adipose tissue. This may be due to an increased rate of general metabolic turnover rather than a specific effect on PAP. Recent research indicates that the hormones most immediately implicated in the control of PAP activity are the glucocorticoids. High serum cortisol concentrations are associated with most of the conditions described above that lead to an increased activity of PAP and an elevated rate of... 

Tijburg LBM, Geelen MJH, van Golde LMG Regulation of the biosynthesis of triacylglycerol, phosphatidylcholine and phos-phatidylethanolamine in the liver. Biochim Biophys Acta 1989 1004 1. 

As precursors for the biosynthesis of fats (lipogenesis), the adipocytes use triacylglycerols from lipoproteins (VLDLs and chylomicrons see p. 278), which are formed in the liver and intestines and delivered by the blood. Lipoprotein lipase [1], which is located on the inner surface of the blood capillaries, cleaves these triacylglycerols into glycerol and fatty acids, which are taken up by the adipocytes and converted back into fats. 

Transfer of an additional acyl residue to DAG forms triacylglycerols (enzyme diacyl-glycerol acyltransferase 2.3.1.20). This completes the biosynthesis of neutral fats. They are packaged into VLDLs by the liver and released into the blood. Finally, they are stored by adipocytes in the form of insoluble fat droplets. 

Acetyl-CoA is oxidized in the TCA cycle and is used in liver and adipose tissue for biosynthesis of fatty acids and triacylglycerol. 

The fate of fatty acids entering the liver is markedly influenced by the ratio of insulin to glucagon in the circulation. High ratios favour esterification of fatty acids into acylglycerols and a low rate of )8-oxidation. Phospholipid biosynthesis is far less affected than that of triacylglycerols. The activity of PAP, however, is probably not directly dependent on insulin since it is increased in diabetes and starvation, both conditions where circulating insulin concentrations are low. 

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