Physiological pH Indicators

The first pH indicators studied possessed the acid-base site (phenol, aniline, or carboxylic acid) as an integral part of the fluorophore. Structurally, in the most general sense, pH sensitivity is due to a reconfiguration of the fluorophorets re-electron system that occurs on protonation. Consequently, the acid and the base forms often show absorption shifts and also, when the two forms fluoresce, emission shifts or at least, when only one form emits, a pH-dependent fluorescence intensity. This class of compounds has been reviewed 112 and the best structures have to be designed according to the medium probed and the technique used. After a short consideration of physiological pH indicators we will describe the main photophysical processes sensible to protonation. 

Some metabolites of curcumin (particularly tetrahydrocurcumin) may also participate in producing the observed effects of curcumin in different models because these metabolites display greater stabilities than the parent curcumin molecule at physiological pH. Recent data show similar modes of action of curcumin metabolites regarding antioxidant enzyme induction and inhibition of multidrug-resistant proteins. " Additional data indicate that curcumin may even act against other types of diseases such as atherosclerosis " " and Alzheimer s disease. " - " ... 

Wolfbeis O. S., Furlinger E., Kroneis H. and Marsoner H. (1983) Fluorimetric Analysis. 1. A Study on Fluorescent Indicators for Measuring Near Neutral ( Physiological ) pH Values, Fresenius Z. Anal. Chem. 314, 119-24. 

Esters ofN,N-dialkylhydroxylamines ((acyloxy)amines) appear to be possible candidates for prodrugs of carboxylic acids, but more studies must be published before any firm conclusion can be drawn. First, there are indications of low acute toxicity for N,N-(1 al ky I hydroxy Iambics [87], Whether the same applies (acutely and chronically) to the pro-moieties after their release from the prodrugs is not known. A second argument is the low basicity of hydroxylamines (the pKa of Ar,AT-dimethylhydroxylamine is 5.2), and the expected lower basicity of O-acylatcd hydroxylamines. As a result, esters of hydroxylamines will be in the unprotonated, more lipophilic form at physiological pH and should be absorbed more readily than the corresponding carboxylic acid [88]. 

Stopped-flow measurements with superoxide in aqueous solution at physiological pH are not possible due to its fast self-dismutation under these conditions. Therefore, the indirect assays such as McCord-Fridovich, adrenalin and nitroblue tetrazolium (NET) assays are widely used in the literature, not only for qualitative but also for quantitative detection of SOD activity of small molecular weight mimetics 52). Not going into details, we just want to stress that the indirect assays have very poor even qualitative reliability, since they can demonstrate the SOD activity of the complexes which does not react with superoxide at all. It has been reported in the literature that this is caused by the interference of hydrogen peroxide 29). We have observed that the direct reaction between complexes and indicator... 

Renal clearance of cotinine is much less than the glomerular filtration rate (Benowitz et al. 2008b). Since cotinine is not appreciably protein bound, this indicates extensive tnbnlar reabsorption. Renal clearance of cotinine can be enhanced by np to 50% with extreme urinary acidification. Cotinine excretion is less influenced by urinary pH than nicotine becanse it is less basic and, therefore, is primarily in the unionized form within the physiological pH range. As is the case for nicotine, the rate of excretion of cotinine is influenced by urinary flow rate. Renal excretion of cotinine is a minor route of elimination, averaging about 12% of total clearance. In contrast, 100% of nicotine Ai -oxide and 63% of 3 -hydroxycotinine are excreted unchanged in the urine (Benowitz and Jacob 2001 Park et al. 1993). 

In a collaboration between the Abelson and Hecht labs [56b], a series of noncoded amino acids were introduced into dihydrofolate reductase (DHFR) to probe substrate binding and the requirement of an aspartic acid residue for catalytic competence. When aspartic acid analogs mono- or disubstituted at the )0-carbon were substituted for the active site aspartic acid residue, the mutant DHFRs were still able to catalyze the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate at 74 - 86 % of the wild-type rate. While hydride transfer from NADPH is not the rate-limiting step for the wild-type enzyme at physiological pH, a kinetic isotope experiment with NADPD indicated that hydride transfer had likely become the rate-limiting step for the mutant containing the )0,)0-dimethylaspartic acid. 

Recently Diederich et al. tried to overcome the solubility problems by using modifications where the linear stearyl side chains are exchanged by branched ones [76]. Dionium salt 53 turned out to be an effective transport catalyst at physiological pH for all investigated compounds [AMP, CTP, 2, 3 -dideoxy-TTP (ddTTP), and 3 -azido-dTTP (AZTTP)] and significantly improved the rates achieved with 52. A chloroform solution of 53 extracts half an equivalent of ATP -, which indicates the formation of a neutral 2 1 complex. Analogously, bis(DABCO) tetracation 54 binds to one ATP molecule. However, the transport acceleration is about one order of magnitude smaller than that of 53 [77]. 

The Zn OH (A), the enzyme is essentially present in this form at the physiological pH, is a relatively good nucleophile and poised for nucleophilic attack on carbon dioxide. Structural studies indicated... 

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