Regulation of lipid and carbohydrate

The phosphorylation and dephosphorylation of seryl, threonyl, and tyrosyl residues regulate the activity of certain enzymes of lipid and carbohydrate metabolism and the properties of proteins that participate in signal transduction cascades. 

This metabolic activity is achieved by a turnover of amino acids and proteins that is as rapid as that of lipids and carbohydrates. In an adult human -400 g of body proteins is turned over each day. Of this -50 g replaces digestive enzymes and -15 g replaces hemoglobin. The amino acid concentration in plasma is small (total 3.2 mM, of which -25% is glutamine) but the turnover of -400 g day of protein is equal to the uptake and release back into the plasma of 4.6 mol of a-amino-iV, so that the average lifetime of an amino acid in the plasma is -5 min. Plasma amino acids are turned over with the same kind of rapidity as plasma glucose or free fatty acids. Like plasma glucose, the plasma amino acid concentration is remarkably constant, but it is not understood how this is regulated. 

Lipogenesis, the synthesis of lipids from carbohydrate via acetyl-CoA, occurs almost exclusively in the liver cells and the fatty tissue, (s. fig. 3.9) According to lipid topogenesis (4), the enzymes involved in triglyceride and phospholipid synthesis are localized on the cytoplasmic surface of the endoplasmic reticulum. The level of hepatic synthesis is regulated primarily by the insulin-glucagon quotient, as described by R.H. Unger in 1971. 

Lipid and carbohydrate metabolism occur to different extents in different organs. The liver regulates the flow of metabolites to brain, muscle, and adipose tissue and ultimately controls the concentration of blood glucose. Adipose tissue is the major storage depot for fatty acids. Triglycerides... 

Thyroid hormone effects on metabolism arc diverse. The rates of protein and carbohydrate synthesis and catabolism are inlluenced. An example of the effect of thyroid hormones on lipid metabolism is the observation of a high serum cholesterol in some hypothyroid patients. This is a consequence of a reduction in cholesterol metabolism due to down regulation of low-density lipoprotein (LDL) receptors on liver cell membranes, with a subsequent failure of sterol excretion via the gut. 

The thyroid gland synthesizes thyroxine (T4) and triiodothyronine (T3), and these hormones are involved in the regulation of growth and development, thermoregulation and calorigenesis, metabolism of carbohydrates, proteins, and lipids, and hypophyseal thyrotropin secretion (see Figure 75). 

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