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Aminohippuric acid

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Aminohippuric acid

Aminohippuric acid or para-aminohippuric acid (PAH), a derivative of hippuric acid, is a diagnostic agent useful in medical tests involving the kidney used in the measurement of renal plasma flow. It is an amide derivative of the amino acid glycine and para-aminobenzoic acid that is not naturally found in man; it needs to be IV infused before use diagnostically.

Uses

Diagnostics

PAH is useful for the measurement of renal plasma flow because it is secreted primarily by the renal tubules; only 20-30% is filtered by the glomerulus.[1] PAH is freely filtered from plasma in the glomerulus and not reabsorbed by the tubules, similar to inulin. PAH differs from inulin in that the fraction of PAH that bypasses the glomerulus and enters the nephron's tubular cells (via the peritubular capillaries) is completely secreted. Thus, through the above two mechanisms, PAH is completely filtered from the plasma reaching the nephron. For this reason, renal clearance of PAH is useful in calculation of renal plasma flow (RPF), which empirically is (1 − hematocrit) × renal blood flow. The clearance of PAH, however, is reflective only of RPF to portions of the kidney that deal with urine formation, and thus underestimates actual RPF by about 10%.[2]

The renal extraction ratio of PAH in a normal individual is approximately 0.92.[3]

Pharmaceuticals

Aminohippuric acid is often used as the sodium salt sodium para-aminohippurate. During World War II, para-aminohippurate was given along with penicillin in order to prolong the time penicillin circulated in the blood. Because both penicillin and para-aminohippurate compete for the same transporter in the kidney, administering para-aminohippurate with penicillin decreased the clearance of penicillin from the body by the kidney, providing better antibacterial therapy. Transporters found in the kidney eliminate organic anions and cations from the blood by moving substances, in this case, drug metabolites, from blood into urine.

Other

In vultures, the NSAID diclofenac, which is extraordinarily toxic to vultures, interferes with the renal transport of uric acid via the PAH channel.[4]

pKa = 3.83

See also

References

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