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A novel enzymatic amperometric method is described for the determination of oxalic acid in urine. An amperometric biosensor was made by immobilizing oxalate oxidase on the surface of a chromium(III) hexacyanoferrate-modified graphite electrode by using a bovine serum albumin and glutaraldehyde cross-linking procedure. The enzyme biocatalyzes oxalate decomposition in the presence of oxygen into carbon dioxide and hydrogen peroxide. The oxalate concentration, which is proportional to the amount of hydrogen peroxide, was determined amperometrically by measuring the current resulting in the reduction of hydrogen peroxide at a very low working potential (0.05 V versus the Hg | Hg2Cl2 | 3M KCl electrode), which minimized the influence of the possible interferences present in human urine. All experiments were performed with succinic buffer, pH 3.8, containing 0.1M KCl and 5.4mM ethylenediaminetetraacetic acid. In an aqueous solution of pure oxalic acid, the biosensor showed good linearity in a concentration range of 2.5–100μM without the use of a dialysis membrane. For untreated urine samples, a high correlation (R2 = 0.9949) was obtained between oxalate concentrations added to urine samples and oxalate recoveries calculated for determinations with the described oxalate biosensor.

In the preceding paper1} we described the chemical deepoxidation of 1 with dissolving metal (Zn). Reductive oxirane cleavage was accompanied by simultaneous allylic rearrangement giving 1 0, 1 3-dihydro-1 3-hydroxy desmycosin (5). It is noteworthy that preparation of 1 consists of two steps2): oxidation of the 12,13-double bond with mchloroperbenzoic acid with simultaneous formation of TV-oxide, and reduction of the TV-oxide with Ph3P.