Pathways

Quinupristin is an antibiotic agent usually used in combination with dalfopristin, to treat Enterococcus faecium bacteremia and skin infections. It inhibits the late phase of protein synthesis by binding to the 50S ribosomal subunit preventing elongation of the forming polypeptide and causing the protein chain to be released prematurely creating a non-functional protein.

Bacterial Autoinducer 2 (AI-2) mediates the quorum sensing 2 system. AI-2 is catalyzed by the luxS enzyme found in both E.coli and S.typhimurium. In E. coli and most pathogenic bacteria that form AI-2 are spontaneous transformations that include cyclization to (2R,4S)-2-methyl-2,4-dihydroxydihydrofuran-3-one and hydration to the final autoinducer (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran. This product is then released from the cell through the AI-2 transporter (tqsA). As the level of AI-2 increases, other cells detect it and import it through the autoinducer-2 ABC transporter (lsrACDB). AI-2 is then degraded in the cells by phosphorylating the AI-2 which is then isomerized to P-HPD. Finally there is a transfer of P-HPD's acetyl group to coenzyme A and the release of dihydroxyacetone phosphates.

Rabeprazole is a drug that belongs to the anti secretory drug class. It is used as an anti-ulcer medication, and helps relieve gastric acid reflux, gastric irritation and gastric pain. It inhibits the proton pump action of ATPase, which blocks the final step of gastric acid secretion. The pathway begins in the parietal cell in the stomach, where rabeprazole and a hydrogen ion use the active metabolite in rabeprazole —rabeprazole thioether — to inhibit potassium-transporting ATPase at the secretory surface of the gastric parietal cell. Now in the gastric endothelial cell, these secretory surfaces are inhibited by rabeprazole and by G-Protein signalling cascade through somatostatin receptor type 4, which is activated by somatostatin. At the same time, potassium-transporting ATPase is activated by the G-protein signalling cascade, through histamine H2 receptor, gastrin/cholecystokinin type B receptor, and muscarinic acetylcholine receptor M3 which are activated by histamine, gastrin and acetylcholine, respectively. The potassium transporting ATPase also converts water and ATP to a phosphate molecule and ADP. Alongside the transporters, potassium is brought into the cell. Carbonic anhydrase 1 uses water and carbon dioxide to create hydrogen carbonate and a hydrogen ion, which are both transported out of the endothelial cell, into the gastric lumen. A chloride ion is transported into the gastric endothelial cell through a chloride anion exchanger and is transported out of the cell through a chloride intracellular channel protein 2, back into the gastric lumen.

Rabeprazole is used for the treatment of acid-reflux disorders (GERD), peptic ulcer disease, H. pylori eradication, and prevention of gastrointestinal bleeds with NSAID use. Rabeprazole is a prodrug administered orally but since it degrades rapidly at low pH, the capsule contains enteric-coated granules. After undergoing absorption in the small intestine, it passes from the blood stream into the parietal cells in the stomach, then enters the stomach lumen. It is a weak base and thus, accumulates on the outside of cell in the acidic environment. Its main target is the H+/K+ ATPase in the parietal cells in the stomach. In parietal cells, carbonic anhydrase converts water and carbon dioxide to hydrogen bicarbonate ions and H+. The bicarbonate ions go into the blood via the chloride anion exchanger on the basolateral membrane which exchanges the hydrogen bicarbonate for Cl- ions. There is also the Na+/K+ ATPase which pumps Na+ out of the cell and K+ into the cell. The H+/K+ ATPase is located on the apical membrane and pumps the H+ from the cell into the stomach lumen and K+ from the lumen into the cell. Another transporter, the K+/Cl- symporter transports K+ and Cl- in the stomach lumen. The H+ and Cl- in the stomach lumen forms the HCl acid which, in excess, can cause disorders like ulcers. The acidic environment in the stomach converts the prodrug Rabeprazole into its active form, sulfenamide. Sulfenamide then covalently binds to the cysteine residues on the alpha subunit of the H+/K+ ATPase via disulfide bridges. This binding of sulfenamide irreversibly inhibits the H+/K+ ATPase, preventing too much acid secretion in the stomach. Less acid in the stomach is favorable for symptomatic relief of disorders caused by the acid. Side effects of taking rabeprazole may include headache, diarrhea, constipation, flatulence, rash, sore throat, infection, stomach pain.