Cyanidase from Bacterial Sources and its Potential for the Construction of Biosensors

Abstract

Because of their content of cyanogenic glycosides, many medicinal and food plants are toxic for man. If plant material containing cyanogenic glycosides gets disintegrated, cyanide is liberated by the action of different enzymes. Especially in developing countries, chronic poisoning by cyanogenic plants is a serious problem. Since probably more than 2500 plant species and also some insects contain cyanogenic glycosides, a rapid and precise method for the determination of these compounds should be developed. A biosensoric system based on an ammonia electrode and the enzyme cyanidase [EC 3.5.5.1] seems to be an effective analytical method for this class of substances and a promising alternative to an ion-selective cyanide electrode. The key-step in the development of such a sensor is the selection of a suitable cyanidase, which has been previously reported for bacteria. This biosensor should be used for screening purposes as well as for the quality control of cyanogenic medicinal and food plants. For this reason, we have examined strains of the bacteria Rhodococcus rhodochrous, Alcaligenes xylosoxidans, and Acinetobacter spec.. Strains were fed with increasing concentrations of potassium cyanide in order to induce cyanidase activity. After three cycles of selection, Alcaligenes xylosoxidans exhibited sufficient growth at cyanide concentrations up to 2´10-3 M. However, Rhodococcus rhodochrous showed excellent performance even at concentrations as high as 1 ´ 10-2 M cyanide. In addition, the latter bacterium is able to digest isovaleronitrile. No significant inhibition of growth was observed at concentrations up to 2 ´ 10-2 M isovaleronitrile. Because of cyanidase activity, Acinetobacter spec. was capable to utilize cyanide as nitrogen source. First measurements with immobilized cyanidase in a flow-through apparatus based on an ammonia electrode gave a detection limit at 0.2 mg/L cyanide. The linear range of the calibration curve was between 0.6 mg/L and 30 mg/L cyanide.