Amino acids, growth limiting

It has been recognized that certain cells have a specific requirement for an amino acid, for example, serine for lymphoblastoid cells (Birch and Hopkins, 1977). This may be due either to the inability of the cells to make an amino acid, or because the amino acid is decomposed in the medium. The concentration of amino acids usually limits the maximum cell concentration attainable, influences cell survival and growth rate, and can affect the synthesis of certain proteins. A too low concentration of an amino acid can result in rapid depletion from the medium, and is thus limiting, whereas a too high concentration can be inhibitory. 

If nitrogen (in the form of ammonia) is growth limiting, the potential applications of acetyl-CoA and NAD(P)H are restricted. Liberated NAD(P)H cannot be consumed for reductive syntheses, for instance of amino acids, it remains available and starts to inhibit citrate synthase [45, 46]. To the extent that the TCA cycle is thereby inhibited, acetyl-CoA should become available for the 3-ketothiolase, and could flow into poly(3HB) (Fig. 1, Table 1). 

A vast number of hormones and neurotransmitters are synthesized from simple structural units including chains of amino acids. Among the amino acid based hormones are comparatively large protein molecules such as prolactin, growth hormone and insulin, and shorter chains of amino acids which may form peptide hormones, such as oxytocin and vasopressin. Protein based hormones are essential for metabolism, growth and some components of reproduction. However, evidence for specific influences of these compounds on human behavior is limited. 

It is a peptide containing 14 amino acids and inhibits the release of growth hormone, TSH and prolactin from the pituitary and insulin and glucagon in pancreas. It has a very short plasma half-life. Because of its shorter duration of action and lack of specificity in inhibiting only GH secretion, its use in the treatment of acromegaly is limited. 

---