Glucuronic acid determination

A.- ng deduction. This is an irreversible reaction which is a foremost determinant of the secretion rate of cortisol (double bonds and C-3 carbonyl). Catalyzed predominantiy by cortisone P-reductase and 3a-hydroxysteroid dehydrogenases, SP sterols result, although 5a sterols are more prevalent in the case of other glucocorticoids. Urocortisol and urocortisone result from the metabohsm of cortisol and cortisone, respectively. Compounds can be complexed to glucuronic acid at this point. 

Although UGTs catalyze only glucuronic acid conjugation, CYPs catalyze a variety of oxidative reactions. Oxidative biotransformations include aromatic and side chain hydroxylation, N-, O-, S-dealkylation, N-oxidation, sulfoxidation, N-hydroxylation, deamination, dehalogenation and desulfation. The majority of these reactions require the formation of radical species this is usually the rate-determining step for the reactivity process [28]. Hence, reactivity contributions are computed for CYPs, but a different computation is performed with the UGT enzyme (as described in Section 12.4.2). 

In determining enzyme activities, it is usually assumed that at a fixed set of so-called saturating substrate concentrations a sufficiently accurate value of F, ax is obtained. Bisubstrate kinetic analyses of UDP-glucu-ronyltransferase [assayed with bilirubin (P5) and p-nitrophenol (V6), respectively] indicate that a true measure of the amount of enzyme can be obtained only by suitable extrapolation procedures. This restriction applies in particular to bilirubin (A2, HIO, T8) and other aglycons (M15, V6) because of substrate inhibition. UDP-glucuronic acid was inhibitory at concentrations only about 10-fold higher than the apparent Km value (HIO) this was most pronounced at relatively short incubation times. Mg was noninhibitory at concentrations equal to 20 times the apparent Km values (F3, HIO). 

The quantitative determination of uronic acids in polysaccharides is complicated by lactonization, and these problems have been carefully examined by Blake and Richards.26,98 In the course of these studies, D-glucuronic acid was reduced to D-glucitol, which was characterized as the hexaacetate.98 As uronic acid residues in a polysaccharide may be reduced relatively easily,465,466 determination as the alditol provides an alternative method of analysis.461,468 This procedure is most valuable when different uronic acids are present in... 

Estriol (estra-1,3,5(10)-trien-3,16-a,17-8 -triol) is the major estrogen metabolite found in the urine. It is excreted in the form of its conjugate with glucuronic acid. The determination of this steroid as an index of placental function has become one of the most widely used endocrine determinations. As pregnancy progresses, the excretion increases and reaches very high levels near term. In abnormal fetoplacental function, the levels of estriol will fall in some cases. The fall is usually progressive, and, because of this, serial determinations of urinary estriol must be carried out. The drop in estriol can be taken as evidence of placental insufficiency, and close watch by the physician is indicated, as a continued drop may necessitate Cesarean section to save the life of the infant. 

The procedure can be used to determine mannuronic and iduronic acids. With glucuronic acid as the standard, the protocol could be used to measure the glucuronic acid contents of xanthan and gellan gums. 

A number of workers have investigated the oxidation of dextrins80 and starch81 with halogens in neutral and alkaline solutions and have reported the presence of significant amounts of anhydroglucuronic acid units in the products obtained, as determined by furfural formation. No attempt has been made, however, to develop a practical synthesis of D-glucuronic acid by this means. 

Free D-glucuronic acid has been found in urine as a consequence of the administration of certain compounds, e. g., aniline,18 phenetidine,10 although they might be expected to form glucuronides in the usual way. It was suggested that the glucuronides actually formed are labile, but their constitution was not determined. 

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