Proteins destined for the plasma membrane are also manufactured in the RER, so that they can be
readily packaged into membrane-bound vesicles and enter the membrane-transport system.
Ribosomes are comprised of two structural subunits: the large subunit and the small subunit.
A ribonucleoprotein is a structure consisting of protein and RNA (ribonucleic acid).
In prokaryotes, the cytoplasm surrounding the nucleoid is rich in risosomes (and is called the riboplasm) - in bacteria proteins that
make up the ribosome are the most abundant proteins in the cytosol.
In eukaryotes ribosomes can exist free in the cytosol or bound to endoplasmic reticulum (forming rough endoplasmic reticulum, or
RER, so-called because the ribosomes stud its outer/cytosolic surface).
AAC is a membrane protein that acts like a revolving door - transporting ADP into mitochondria (to be converted to ATP) and ATP out of mitochondria and into the cytoplasm (Wang and Tajkhorshid 2008).
Initially, the membrane transport protein (also called a carrier)is in its closed configuration which does not allow substrates or othermolecules to enter or leave the cell.
A system of preparation of rat hepatocytes with extended viability has been developed to study the role of hormones and other plasma components upon secretory protein synthesis. Hepatocytes maintained in minimal essential medium reduced the levels of all amino acids in the medium except the slowly catabolized amino acids leucine, isoleucine, and valine, which steadily increase as the result of catabolism of liver protein. Although the liver cells catabolize 10-15% of their own protein during a 20-h incubation, the cells continue to secrete protein in a linear fashion throughout the period. The effects of insulin, cortisol, and epinephrine on general protein synthesis, and specifically on fibrinogen and albumin synthesis, have been tested on cells from both normal rats and adrenalectomized rats. Cells from normal animals show preinduction of tyrosine amino transferase (TAT), having at the time of isolation a high level of enzyme which shows only an increase of approximately 60% upon incubation with cortisol. In contrast, cells from adrenalectomized animals initially have a low level of enzyme which increases fourfold over a period of 9 h. The effects of both epinephrine and cortisol on protein synthesis are also much larger in cells from adrenalectomized animals. After a delay of several hours, cortisol increases fibrinogen synthesis sharply, so that at the end of the 20-h incubation, cells treated with hormone have secreted nearly 2.5 times as much fibrinogen as control cells. The effect is specific; cortisol stimulates neither albumin secretion nor intracellular protein synthesis. The combination of cortisol and epinephrine strongly depresses albumin synthesis in both types of cells. Insulin enhances albumin and general protein synthesis but has little effect on fibrinogen synthesis.
We presently compared synthesis rates of fibrinogen and albumin in nephrotic adults (N = 7; plasma albumin 22.3 +/- 0.7 g/liter, proteinuria 12 g/day) and healthy control subjects (N = 8) using a primed/continuous infusion of the stable isotope L-[1-C-13]valine for six hours.
Protein C exerts a potent anticoagulant effect by inhibiting thrombin generation. Activated Protein C, in a reaction accelerated by Protein S, cleaves arginine sites on Factor Va and Factor VIIIa, to produce degradation fragments with no cofactor activity. The lack of these factors prevents rapid production of Factor Xa and thrombin to drive the amplification and propagation phases of coagulation that generate a large burst of thrombin (). This proteolytic inactivation of Factor Va and Factor VIIIa is a critical regulatory mechanism that prevents pathologic thrombosis and thereby maintains blood flow.
Protein C is a vitamin K-dependent serine protease anticoagulant that is structurally similar to the vitamin K-dependent coagulation factors (Factors II, VII, IX, X). These proteins are synthesized primarily in the liver and all require post-translational gamma carboxylation to bind Ca++ and attain a fully functional form. Protein C circulates as a zymogen that is activated at the surface of endothelial cells by interacting with thrombin bound to its endothelial cell receptor, thrombomodulin (TM). This Protein C –thrombin interaction is greatly enhanced by Protein C’s binding to endothelial cell protein C receptor (EPCR). Activated Protein C (APC) is released from the EPCR and then combines with its cofactor, Protein S, on the phospholipid surface of endothelial cells and platelets. Protein C zymogen circulates in plasma with a half-life of approximately 6 hours. The protease APC circulates in only trace amounts where it is rapidly inactivated (within minutes) by plasma inhibitors.
Acquired Protein C deficiency develops much more commonly in veterinary medicine, often in association with liver disease. In particular, severe Protein C deficiency develops in patients with liver failure and congenital portosystemic vascular anomalies (PSVA). Protein C’s short plasma half-life likely contributes to the development of Protein C deficiency secondary to hepatic synthetic failure. The etiopathogenesis of Protein C deficiency secondary to portosystemic shunting is not yet well-defined. In addition to liver disease, vitamin K deficiency of any cause will cause a functional Protein C defect due to impaired post-translational processing and an inability of Protein C to interact with coagulation complexes.
The Protein C assay performed at the Comparative Coagulation Laboratory for clinical diagnosis is a functional measure of plasma protein C levels (Protein C activity assay). The test plasma is treated with a venom-derived activator and the proteolytic action of Protein C is then measured based on the enzyme’s cleavage of a chromogenic substrate. This is a fluid phase, commercial assay that does not evaluate Protein C’s interaction with cell membrane receptors (thrombomodulin or EPCR) and does not measure circulating APC
Citrate plasma is the only acceptable specimen-type for the Protein C activity assay. At least 0.5 mL separated citrate plasma should be shipped on cold packs for overnight delivery. See for more sampling details.
Protein C is a major plasma anticoagulant that acts at the surface of endothelial cells. Activated Protein C neutralizes Factors Va and VIIIa, which effectively blocks the amplification and propagation of coagulation. Protein C is a vitamin K-dependent protein, synthesized in the liver with a short plasma half-life. Protein C assays aid in the diagnosis of liver disease and thrombotic disorders. Clinical studies of dogs with congenital hepatoportal vascular anomalies indicate that Protein C is a non-invasive measure of portal blood flow, useful for differentiating portosystemic shunting (PSS) from microvascular dysplasia (MVD).