PubMed · 8295134
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S Marr, H F Molloy. 1994. Sock durability.. https://doi.org/10.7547/87507315-84-1-46
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The angiotensin II (AII) antagonist, losartan, increases uric acid excretion when administered to humans. However, the active metabolite of losartan, EXP 3174, and other nonpeptide AII antagonists such as eprosartan and SB 203220 are devoid of uricosuric activity. To investigate the mechanism of losartan-induced uricosuria, we examined the effects of losartan, EXP 3174, eprosartan and SB 203220 on OH- -dependent [14C]urate uptake into rat proximal tubule brush-border membrane vesicles. Losartan (10 microM) inhibited [14C]urate uptake at all time points examined, except at equilibrium (2 hr). Losartan had no effect on urate uptake in the absence of an OH- gradient. The inhibitory effect of losartan on urate uptake was concentration dependent (IC50 = 9.5 +/- 1.4 microM) and competitive in nature. The other AII antagonists also inhibited urate uptake but were 6-8-fold less potent than losartan with IC50 values of EXP 3174 (65 +/- 13 microM), eprosartan (60 +/- 7.0 microM) and SB 203220 (74 +/- 12.5 microM). In contrast to the effects of the nonpeptide AII antagonists, the peptide antagonist, Sar1,Ile8-AII, as well as AII itself had no effect on urate uptake. These results suggest that the uricosuric activity of losartan is, at least in part, due to inhibition of urate reabsorption in the proximal tubule and is unrelated to AII receptor activity. Furthermore, losartan has a greater affinity for the urate/anion exchanger than the other AII antagonists tested. These results are in direct agreement with observations made after administration of these compounds to humans.
The pyruvate produced by glycolysis in the bloodstream form of the trypanosome is excreted into the host bloodstream by a facilitated diffusion carrier. The sensitivity of pyruvate transport for alpha-cyano-4-hydroxycinnamate and the compound UK5099 [alpha-cyano-beta-(1-phenylindol-3-yl)acrylate], which are known to be selective inhibitors of pyruvate (monocarboxylate) transporters present in mitochondria and the plasma membrane of eukaryotic cells, was examined. The trypanosomal pyruvate carrier was found to be rather insensitive to inhibition by alpha-cyano-4-hydroxycinnamate (Ki = 17 mM) but could be completely blocked by UK5099 (Ki = 49 microM). Inhibition of pyruvate transport resulted in the retention, and concomitant accumulation, of pyruvate within the trypanosomes, causing acidification of the cytosol and osmotic destabilization of the cells. Our results indicate that this physiological state has serious metabolic consequences and ultimately leads to cell death; thereby identifying the pyruvate carrier as a possible target for chemotherapeutic intervention.
To investigate the mechanism by which the tryptophan synthase beta subunit accelerates the cleavage of the indole-3-glycerol phosphate catalyzed by the alpha subunit (alpha reaction), kinetic experiments were carried out with wild-type and mutant forms of the alpha 2 beta 2 complex. Previous studies indicate that this activation can be attributed to the conformational changes associated with the formation of a Schiff base between aminoacrylate and pyridoxal phosphate at the beta site. To test this hypothesis, we investigated a mutant form of the alpha 2 beta 2 complex having the lysine-87 of its beta subunits replaced by threonine. The mutant alpha 2 beta 2 complex (K87T) exhibits normal activity for the alpha reaction but fails to catalyze formation of L-tryptophan from L-serine and indole (beta reaction). However, the mutant enzyme can form a Schiff base intermediate with L-serine at the beta site. Using a "chemical rescue" method, we converted K87T L-serine intermediate to an aminoacrylate intermediate. Steady-state kinetic studies reveal that the aminoacrylate derivative exhibits a 7-fold enhancement in kcat/Km for the alpha reaction relative to that of the L-serine derivative of the mutant or the wild-type enzyme in the absence of L-serine. Rapid kinetic data show that the aminoacrylate derivative of the mutant enzyme exhibits a 6-fold increase in the rate constant for the indole-3-glycerol phosphate cleavage reaction. In addition, rate constants for the reverse reaction and product release steps are also altered. Together, these changes lead to a decrease in Km and an increase in kcat.(ABSTRACT TRUNCATED AT 250 WORDS)