The substrate [2,2'-azino-bis(3-ethylbenzthiazoline sulfonicacid)] (Sigma) was diluted in citrate buffer (pH 4) to a final concentration of 210 pug/ml, with 0.1 ml of 0.5 M H202 addedto each 25 ml of substrate solution. The A405 was read threetimes at 2-min intervals with an ELISA reader (TitertechMultiscan Mark II; Flow Laboratories).