Amperometric enzyme electrode for the determination of urine oxalate

An enzyme electrode has been developed for the simplified determination of oxalate in human urine. Oxalate oxidase has been co-immobilised with bovine serum albumin (BSA) using glutaraldehyde between polycarbonate or haemodialysis external membranes and an internal cellulose acetate membrane to form a classical oxidase enzyme laminate construction. The underlying cellulose acetate permselective barrier confers the required selectivity for H2O2 over the interferent constituents of urine. Prior dilution and acidification of urine samples has enabled optimisation of enzyme activity and therefore electrode response characteristics for low level oxalate assay. Sequestering of singly and doubly charged cations in urine by ethylenediaminetetraacetic acid (EDTA) has ensured release of bound oxalate, so permitting near total (> 98%) determination of urinary oxalate. An outer substrate diffusion limiting membrane has proved critical to an electrode linear response within the clinical concentration range. Determination of human urinary oxalate in the concentration range 2–200 μM in diluted urine (corresponding to 80–800 μM oxalate in undiluted urine) has been possible. Results correlate well (r2 = 0.971; y = 1.11x + 4.20; n = 20) with standard spectrophotometric determinations (x) performed within a hospital laboratory.