1. Characterization of p-hydroxy-hippuric acid as an inhibitor of Ca2+-ATPase in end-stage renal failure
J Jankowski, M Tepel, N Stephan, M van der Giet, V Breden, W Zidek, H Schlüter Kidney Int Suppl. 2001 Feb;78:S84-8. doi: 10.1046/j.1523-1755.2001.59780084.x.
Characterization of p-hydroxy-hippuric acid as an inhibitor of Ca2+-ATPase in end-stage renal failure. In patients with end-stage renal failure (ESRF), disturbances of Ca2+ metabolism are common. Besides hormonal changes, inhibition of cellular Ca2+-ATPase was postulated to contribute to uremic toxicity. We purified a potent inhibitor of the Ca2+-ATPase from the ultrafiltrate of patients with ESRF by multiple steps of high-performance liquid chromatography to homogeneity, and identified the isolated inhibitor by mass spectrometric methods as p-hydroxy-hippuric acid. The enzyme used for the Ca2+-ATPase assay system was isolated from red blood cells by cross-flow filtration. The activity of the Ca2+-ATPase was measured spectrophotometrically as the difference in hydrolysis of adenosine 5'-triphosphate (ATP) in the presence and absence of Ca2+ with different concentrations of ATP and p-hydroxyhippuric acid. The Ca2+-ATPase was found to be inhibited by p-hydroxy-hippuric acid at a concentration above 11.7 micromol/L. p-Hydroxyhippuric acid inhibited the erythrocyte Ca2+-ATPase by reducing Vmax and increasing the Km value. The EC50 (log mol/L; mean +/- SEM) for p-hydroxy-hippuric acid was calculated as 4.82 +/- 0.14. In conclusion, p-hydroxy-hippuric acid may play a role in disturbed Ca2+ metabolism in end-stage renal failure.
2. Uraemic toxins induce proximal tubular injury via organic anion transporter 1-mediated uptake
Masaru Motojima, Atsuko Hosokawa, Hideyuki Yamato, Takamura Muraki, Toshimasa Yoshioka Br J Pharmacol. 2002 Jan;135(2):555-63. doi: 10.1038/sj.bjp.0704482.
A direct effect of uraemic toxins in promoting progression of chronic renal disease has not been established. In this study, we investigated the toxic effects of organic anions which characteristically appeared in the patients with progressive renal disease on renal proximal tubular cells expressing human organic anion transporter (hOAT) 1. A renal proximal tubular cell line, opossum kidney (OK) cells, was transformed with hOAT1. Among the organic anions examined, hippuric acid, para-hydroxyhippuric acid, ortho-hydroxyhippuric acid, indoxyl sulphate and indoleacetic acid showed a high affinity for hOAT1 expressed in the OK cells. Indoxyl sulphate and indoleacetic acid concentration-dependently inhibited proliferation of the hOAT1-transformed cells. The h.p.l.c. analysis demonstrated that cellular uptake of these organic anions was significantly elevated in hOAT1-transformed cells. These organic anions also concentration-dependently stimulated cellular free radical production. The degrees of inhibition of cell proliferation and the stimulation of free radical production induced by the organic anions were significantly higher in the hOAT1-transformed cells than vector-transformed cells. The stimulatory effect of indoxyl sulphate on free radical production was abolished by anti-oxidants and probenecid. Less free radical production was observed in the hOAT1-transformed cells treated with p-hydroxyhippuric acid, o-hydroxyhippuric acid compared with indoxyl sulphate and indoleacetic acid. Hippuric acid had little effect on free radical production. Organic anions present in the serum of patients with progressive renal disease may cause proximal tubular injury via hOAT1-mediated uptake. The mechanism of cellular toxicity by these uraemic toxins involves free radical production. Thus, some uraemic toxins may directly promote progression of chronic renal disease.
3. The study on the biological fate of paraben at the dose of practical usage in rat. II. The pharmacokinetic study on the blood concentration after the administration of ethyl paraben or p-hydroxybenzoic acid
H Kiwada, S Awazu, M Hanano J Pharmacobiodyn. 1980 Jul;3(7):353-63. doi: 10.1248/bpb1978.3.353.
The biological fates of ethyl paraben and p-hydroxybenzoic acid in rat were investigated after intravenous and intraduodenal administrations at the dose of 2 mg/kg. The blood concentrations were measured in detail from 3 min after the administration at appropriate time intervals until 90 min. Areas under the blood concentration curves and clearances were calculated from these time course data. Ethyl paraben was little detected in blood after intraduodenal administration. It is suggested that the intestinal metabolism and the first pass effect in liver greatly contribute to the hydrolysis of ethyl paraben. Total radio activity after intraduodenal administration did not show the maximum peak and decreased rapidly. The maximum peak was not observed also in the time course of p-hydroxyhippuric acid after intravenous administration. It shows that not only the rate of hydrolysis but also absorption and conjugation are very rapid. The differences of the areas under the blood concentration curves of p-hydroxybenzoic acid or p-hydroxyhippuric acid was found between the routes or chemical forms of administration. The complex kinetic mechanisms were assumed in the biological fates of these compounds as follows: Conjugation to p-hydroxyhippuric acid is excellent in ethyl paraben administration than p-hydroxybenzoic acid, and in intraduodenal administration than intravenous administration. These phenomenon can not be explained by the conventional kinetic model which is constructed with the connected blood compartments in series. The kinetic models including the assumed routes conjugating directly ethyl paraben in blood or intestine to p-hydroxyhippuric acid were presented, and the least square curve fitting analyses were carried out on these kinetic models.
