Paraoxonase activity was determined spectrophotometrically using paraoxon (O, O-diethyl-o-p-nitro-phenylphosphate; Sigma Chemical Co) as the substrate and measured by increases in the absorbance at 412 nm due to the formation of 4-nitrophenol as already described [23 (link)]. Briefly, the activity was measured at 25°C by adding 50 μl of serum to 1 ml Tris-HCl buffer (100 nM at pH 8.0) containing 2 mM CaCl2 and 5.5 mM of paraoxon. The rate of generation of 4-nitrophenol was determined at 412 nm with a spectrophotometer (Techcomp 8500 II UV/VIS, China). PON1 activity is expressed in U/l serum. One unit of PON1 activity was defined as 1 nmol of 4-nitrophenol formed per minute under the above assay conditions.
Arylesterase activity was also measured spectrophotometrically using phenylacetate (Sigma Co, London, UK) as the substrate. The phenol formed after the addition of a 40-fold diluted serum sample was measured spectrophotometrically at 217 nm following an established procedure [24 (link)]. The activity of ARE was expressed in kU/l serum. One unit was defined as the enzyme quantity that disintegrates 1 nmol phenylacetate per minute.
Arylesterase activity was also measured spectrophotometrically using phenylacetate (Sigma Co, London, UK) as the substrate. The phenol formed after the addition of a 40-fold diluted serum sample was measured spectrophotometrically at 217 nm following an established procedure [24 (link)]. The activity of ARE was expressed in kU/l serum. One unit was defined as the enzyme quantity that disintegrates 1 nmol phenylacetate per minute.
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