FLUORIDE DETERMINATION BY ITS INHIBITORY EFFECT ON IMMOBILIZED LIVER ESTERASE

The inhibition of liver esterase (EC 3.1.1.1.) immobilised on controll ed pore glass by fluoride ions was studied in a now injection system. Ethyl butyrate was used as substrate, ethanol and butyric acid being p roduced as hydrolysis products. To monitor the reaction a second enzym atic reaction was employed, in which alcohol dehydrogenase, immobilise d on controlled pore glass, catalysed the oxidation of ethanol and red uction of the coenzyme, NAD(+), to NADH, the latter being determined s pectrophotometrically at 340 nm. Fluoride inhibition of the immobilise d liver esterase was fast and reversible, and there was a linear relat ionship between %inhibition and fluoride concentration over the range 8 X 10(-7)-8 X 10(-6) M (15-150 mu g l(-1)) fluoride. The relative sta ndard deviation at mid-calibration range is below 4% (n = 5). The more common inorganic compounds present in water samples do not interfere in the fluoride determination.