disulfide bonds. This action fundamentally alters the configuration of the acid pump, thereby inhibiting its activity. Thus, acid can no longer be secreted into the gastric lumen (the empty space of the stomach), and the pH of the stomach increases (decrease in the concentration of hydrogen ions, H+). Due to the permanent inhibition of the individual proton pump that each molecule of rabeprazole has bound to, acid secretion is effectively suppressed until new proton pumps are produced by the parietal cells. Rabeprazole, like other medications in the same class, cannot inhibit the H+/K+ ATPase pumps found in lysosomes, a cellular organelle that degrades biological molecules, because the pumps found in these organelles lack the cysteine residues involved in rabeprazole's mechanism of action. A unique feature of rabeprazole's mechanism of action in inhibiting acid secretion involves its activation. The pKa (the pH at which 50% of the drug becomes positively charged) of rabeprazole is around 5.0, meaning that it doesn't take a lot of acid to activate it. While this theoretically translates into a faster onset of action for rabeprazole's acid-inhibiting effect, the clinical implications of this fact have yet to be elucidated. Pharmacokinetics Rabeprazole's bioavailability is approximately 52%, meaning that 52% of orally administered dose is expected to enter systemic circulation (the bloodstream). Once in the blood, rabeprazole is approximately 96.3%-97% bound to plasma proteins. The biological half-life of rabeprazole in humans is approximately one hour. It takes about 3.5 hours for rabeprazole to reach the maximum concentration in human plasma after a single orally administered dose. Oral absorption is independent of the dose administered. Rabeprazole is extensively metabolized by the liver. 90% of the drug is rendered into metabolites by the liver, which are then excreted by the kidneys. 10% of the dose is excreted in the feces. The drug metabolizing enzymes primarily responsible for rabeprazole's metabolism are CYP2C19 and CYP3A4. However, rabeprazole is mainly metabolized through non-enzymatic reduction to a thioether metabolite. Some of rabeprazole's metabolites include the following: a thioether carboxylic acid metabolite, a thioether glucuronide metabolite, and a sulfone metabolite. The most common metabolites excreted in the urine are the mercapturic acid conjugate and carboxylic acid. A diagram of rabeprazole's phase I metabolism is shown below. Pharmacogenetics The effect of rabeprazole may vary based upon the genetics of the individual taking the medication. People may have differences in their capacity to metabolize rabeprazole to an inactive