Uric acid

A 1 mM standard BLM solution was prepared by dissolving an appropriate amount of bleomycin sulfate (Merck, Darmstadt, Germany) in a 0.9% NaCl (saline) solution. This BLM solution was further diluted with saline to obtain BLM solution with a concentration of 10 µM. Sodium acetate and acetic acid used to make an acetate buffer (NaAc—HAc) of pH 4.5, which acted as the supporting electrolyte, were purchased from Merck. There were 1 mM stock solutions of Ni(II), Cd(II), Ca(II), V(V), Fe(III), Mg(II), Cu(II), glucose, ascorbic acid, dopamine, adenine, epinephrine, uric acid and testosterone that were prepared from Merck reagents in deionized water or ethanol (testosterone) and stored at 4 °C in the dark until the influence of interferents was examined. Diethylenetriaminepentaacetic acid (DTPA) was purchased from Merck. Ultra-purified water from a Milli-Q system (Millipore, Livingston, Scotland, UK) was used to prepare the solutions.

MSU crystals were produced as described previously [10 (link), 19 (link)]. We dissolved 10 mM uric acid (Merck KGaA) and 154 mM NaCl (Merck KGaA) to pH 7.2 and stirred it for 3d. We dried the resulting crystals under sterile conditions at 180 °C for 2 h after ethanol washing, and stored them in PBS (pH 7.0).

To 1.0 g of freeze-dried homogenized sample, 5 mL of milliQ water was added and mixed thoroughly. Then 1.00 g of the sample mixture was transferred to another tube to which 2.5 mL of milliQ water, followed by 0.2 mL of 0.5 mol/L H₂SO₄ (Merck) and 8 mL of acetonitrile (Merck) were added. Mixing was done for 30 min on a MultiRS-60 BIOSAN rotator (Nerlien, Oslo, Norway). The rest of the procedure was done according to Narvhus et al. (1998 (link)). Organic acids, glucose, fructose, and maltose levels were analyzed by HPLC. The organic acids were detected with a UV detector set at 210 nm and the sugars were determined using a refractive index detector (Perkin Elmer series 200, Norwalk, CT). Organic acids were identified based on comparison of their retention times with standard solutions of citrate, orotic acid, pyruvate, succinate, dl-lactate, uric acid, dl-pyroglutamate, propionate, α-ketoglutaric acid, oxalic acid, acetate, and formate (Merck). Identification of sugars was also based on retention times of standard solutions of maltose, lactose, galactose, fructose, and glucose (Merck). Quantification was done using external calibration curves of mixed standards in deionized water.

Flaxseed oil cake (FOC) was purchased from ACS Sp. z o.o. (Bydgoszcz, Poland). The proximate composition of FOC (based on supplier information) was: solids—80.50%, including: proteins—41.97%; carbohydrates—27.99%; fiber—6.29%; fat—6.11%; ash—4.50%. Lacticaseibacillus rhamnosus GG (ATCC53103), was procured from ATCC (Manassas, VA, USA). Buffered peptone water, microbiological agar, MRS agar and broth were purchased from Oxoid (Basingstoke, UK). Potassium chloride, sodium chloride, potassium thiocyanate, disodium hydrogen phosphate, monosodium dihydrogen orthophosphate, calcium chloride, sodium hydrogen carbonate, hydrochloric acid, ammonium chloride, sodium peroxide, urea, α-amylase, uric acid, mucin from porcine stomach (type II), glucose, glucuronic acid, glucosamine hydrochloride, bovine serum albumin (BSA), pepsin, pancreatin, oxgall, Triton X-100, cholesterol, ethanol, ninhydrin, glacial acetic acid, cadmium chloride and hexadecane were purchased from Merck Chemical (Saint Louis, MI, USA). All reagents were of analytical grade.

High purity Trimethylamine N-Oxide (TMAO) was procured from SigmaAldrich. Pyrrole, glucose, uric acid, and urea were purchased from Merck. Ferric chloride (FeCl₃), hydrochloric acid, ethanol, and sodium chloride (NaCl) were procured from SRL Limited. Sodium hydroxide pellets (NaOH), sodium phosphate monobasic anhydrous (NaH₂PO₄), sodium phosphate dibasic dihydrate (Na₂HPO₄) potassium ferricyanide (K₃[Fe(CN)₆]), potassium ferrocyanide (K₄[Fe(CN)₆]3H₂O) were obtained from fisher scientific. De-ionized water (DI) from the Millipore water purification system was used to prepare the solutions. The ferri-ferro containing phosphate buffer at different pH were made in the lab, as described in the previous report^{53 (link),63 (link)}. Indium Tin Oxide (ITO) coated glass substrate of 1.1 mm thickness, having a transmittance of 90%, and sheet resistance of 25 Ω sq⁻¹ was purchased fromBlazers (U.K). All other chemicals were used without any further purification and were of analytical grade. TMAO solution was prepared at various concentrations from a specific stock solution of TMAO in the range of 0.1 – 15 ppm [0.1 ppm, 0.5 ppm, 1 ppm, 2 ppm, 4 ppm, 6 ppm, 8 ppm, 10 ppm, 12 ppm, and 15 ppm] for the electrochemical response study experiments.

Thymidine, hypoxanthine, sodium formate, adenine, adenosine, guanosine, deoxyuridine, H₂O₂ and pyrazofurin are from Sigma Aldrich. AICAr is from Chemtronica. Febuxostat is from Stratech Scientific Ltd. Allopurinol and Uric acid were from Merck Life Science UK.