Hormonal Regulation of Protein Metabolism
Growth Hormone Increases the Synthesis of Cellular Proteins.
Growth hormone causes the tissue proteins to increase. The precise mechanism by which this occurs is not known, but it is believed to result mainly from increased transport of amino acids through the cell membranes or acceleration of the DNA and RNA transcription and translation processes for protein synthesis.
Insulin Is Necessary for Protein Synthesis. Total lack ofinsulin reduces protein synthesis to almost zero. The mechanism by which this occurs is also unknown, but insulin does accelerate the transport of some amino acids into cells, which could be the stimulus to protein synthesis. Also, insulin increases the availability of glucose to the cells, so that the need for amino acids for energy is correspondingly reduced.
Glucocorticoids Increase Breakdown of Most Tissue Proteins.
The glucocorticoids secreted by the adrenal cortex decrease the quantity of protein in most tissues whileincreasing the amino acid concentration in the plasma, as well as increasing both liver proteins and plasma pro-teins. It is believed that the glucocorticoids act byincreasing the rate of breakdown of extrahepatic pro-teins, thereby making increased quantities of amino acids available in the body fluids. This supposedly allows the liver to synthesize increased quantities of hepatic cellular proteins and plasma proteins.
Testosterone Increases Protein Deposition in Tissues. Testos-terone, the male sex hormone, causes increased deposi-tion of protein in tissues throughout the body, especially the contractile proteins of the muscles (30 to 50 per cent increase). The mechanism of this effect is unknown, but it is definitely different from the effect of growth hormone, in the following way: Growth hormone causes tissues to continue growing almost indefinitely, whereas testosterone causes the muscles and, to a much lesser extent, some other protein tissues to enlarge for only several months. Once the muscles and other protein tissues have reached a maximum, despite continued administration of testosterone, further protein deposi-tion ceases.
Estrogen. Estrogen, the principal female sex hormone,also causes some deposition of protein, but its effect is relatively insignificant in comparison with that of testosterone.
Thyroxine. Thyroxine increases the rate of metabolism ofall cells and, as a result, indirectly affects protein metab-olism. If insufficient carbohydrates and fats are avail-able for energy, thyroxine causes rapid degradation of proteins and uses them for energy. Conversely, if ade-quate quantities of carbohydrates and fats are available and excess amino acids are also available in the extra-cellular fluid, thyroxine can actually increase the rate of protein synthesis. In growing animals or human beings, deficiency of thyroxine causes growth to be greatly inhibited because of lack of protein synthesis. In essence, it is believed that thyroxine has little specific effect on protein metabolism but does have an important general effect by increasing the rates of both normal anabolic and normal catabolic protein reactions.