Use of an optical biosensor with a silicone-immobilized enzyme to determine plasma total cholesterol concentrations in fish

Decreased plasma total cholesterol concentrations are a useful indicator in fish of reduced resistance to bacterial infection. Taking advantage of the hydrophobic properties of silicone rubber, we developed an optical enzyme sensor system for measuring plasma cholesterol concentrations in fish. The system was constructed using an immobilized enzyme membrane, optical oxygen probe, flow system, and personal computer. The enzyme membrane was prepared from cholesterol esterase, cholesterol oxidase, and silicone rubber. The calibration curves were linear in the range of 55–85 mg dl⁻¹ for yellowtail and 20–90 mg dl⁻¹ for rainbow trout. Assays could be completed within 3 min, and the sensor response was stabilized by the hydrophobic properties of silicone rubber. Storage of immobilized enzyme membranes at 5°C permitted stable measurements for 58 days. The sensor system was applied to determine plasma total cholesterol concentrations in fish. Good correlations were obtained between results obtained using the sensor and results obtained using conventional methods (correlation coefficients: yellowtail 0.9751, rainbow trout 0.9947). Our method required much less time than conventional cholesterol assays and can be used economically for continuous determination of plasma total cholesterol in fish.