Hydroquinone assay

Metabolism. Decoction of the seed, administered orally to adults at variable doses, was active. Volunteers consumed food containing hydroquinone or glycopyranoside derivative (arbutin). Blood and urine levels of the compounds and conjugates were assayed . 

Hydroquinone w as tested for carcinogenicity in two studies in mice and two studies in rats by oral administration. It was also tested in rats for promoting activity in assays for bladder, stomach, liver, lung, oesophagus and kidney carcinogenesis and in one study in hamsters for pancreatic carcinogenesis. 

Hydroquinone had no promoting activity in most assays an increase in the multiplicity of oesophageal tumours was obsers ed in one study and in the multiplicity of renal cell tumours in another study. No promoting effect on pancreatic carcinogenesis was observed in the study in hamsters. 

Adler, I.-D. Kliesch, U. (1990) Comparison of single and multiple treatment regimens in the mouse bone marrow micronucleus assay for hydroquinone (HQ) and cyclophosphamide (CP). Mutat. Res., 234, 115-12,3... 

Table V. Molar Absorptivities in the Total Phenol Assay and Precipitability with Acidic Formaldehyde Hydroquinone Derivatives...

An assay for the determination of vitamin K3 (2-methyl-1,4-naphthoquinone) by a combination of both constant-potential and constant-current cou-lometry has been reported [13]. The assay requires the two-electron reduction of the compound to the corresponding hydroquinone at a mercury pool electrode (E = -0.60 V vs. SCE) in acetate buffer, pH 5.9, followed by the coulometric titration of the reduction product with electrogenerated Ce(VI). This method is preferable to the standard method requiring preliminary reduction to the hydroquinone by zinc dust in acid medium, followed by titration with standard Ce(IV) solution. It is capable of low-level determination (1-2 mg) of this vitamin in pharmaceuticals, biological fluids, and foods. 

Tosylactivated magnetic beads (MB-Tosyl) (Dynabeads M-280 Prod. No. 142.03, Dynal Biotech ASA, Oslo, Norway). Hydroquinone (Sigma), hydrogen peroxide (Merck, Germany). Bradford solution (Bio-Rad protein assay Catalog No. 500-0006) was purchased from Bio-Rad laboratories GMGH (Munich, Germany). 

Sample preparation for complex formulations, such as creams, can frequently be as simple as dissolving the cream in the totally organic mobile phase such as the ones typically used in normal-phase chromatography. Organic solutions of flurometholone [3, p. 677] and hydrocortisone acetate [3, p. 758] creams were assayed by HPLC and hydroquinone cream by TLC [3, p. 769]. A similiar approach has been applied to sample preparation of ointments. 

Assay Not less than 99.0% of C10H14O2. fe/ f-Butyl-p-benzoquinone Not more than 0.2%. 2,5-Di-ferf-butylhydroquinone Not more than 0.2%. Hydroquinone Not more than 0.1%. 

Assay Transfer about 170 mg of sample, previously ground to a fine powder and accurately weighed, into a 250-mL wide-mouth Erlenmeyer flask, and dissolve in 10 mL of methanol. Add 150 mL of water, 1 mL of 1 N sulfuric acid, and 4 drops of diphenylamine indicator (3 mg of / -di phenyl ami ncsu I Ionic acid sodium salt per milliliter of 0.1 N sulfuric acid), and titrate with 0.1 N ceric sulfate to the first complete color change from yellow to red-violet. Record the volume, in milliliters, of 0.1 N ceric sulfate required as V. Calculate the percentage of C10H14O2 in the sample, uncorrected for hydroquinone and 2,5-di-tm-butylhydroquinone, by the formula... 

Silver can be isolated along with the platinum metals by fire assay, with copper(I) sulphide as collector [22]. Silver (and gold) present in the sample are collected in copper on melting at 1,200 C. After dissolving the copper alloy and diluting the obtained solution, silver (and gold) are reduced to the metals with formic acid or hydroquinone [23]. 

After confirmation in the radiochemical assay, one compound was finally retained for further evaluation. It was identified as a known commercially available hydroquinone derivative (Salor S6.721-1, Table 1). The compound had an IC50 value of 2 pM, when preincubated with the enzyme for 10 min. Structures lacking either the hydroxyl or dimethylaminomethyl groups were not active (Table 1). No inhibition by S6,721-l was observed in the presence of 1 mg/ml ascorbate. Inhibition was time-dependent (Fig. 4) and could not be reversed by extensive dialysis. The presence of substrate in the reaction mixture protected the enzyme from inactivation (data not shown). This indicates that one or several active site residues are irreversibly modified by the hydroquinone derivative. A mutant enzyme, where Lys-36 in the active site had been replaced with glutamine (8) was not inactivated, suggesting that Lys-36 may be the modified residue. Interestingly, 56,721-1 was most effective after a short incubation time in pH 7.5 buffer alone, before addition of enzyme and finally substrate (Fig. 4). However, longer pre-incubation times under these conditions partially destroyed the inhibitory activity (Fig. 4) and after 1 hour at pH 7.5 the compound was completely inactive. 

In Ames and micronucleus tests of urine from human volunteers administered the compound arbutin at doses of 420 mg, no mutagenic or genotoxic activity was observed (Siegers et al. 1997). No mutagenicity of arbutin was observed in a gene mutation assay at a concentration of 0.01 M, whereas the compound hydroquinone caused an increase in mutation frequency at a concentration of 0.01 M (Mueller and Kasper 1996). 

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