Inhibition of ascorbate autoxidation by a dialyzed, heat-denatured extract of plant tissues

Preparations obtained from various plant sources were analyzed for their effect on the autoxidation of ascorbic acid and norepinephrine. The former reaction was followed by spectrophotometric detection of ascorbic acid at 265 nm, the latter one by measuring the formation of noradrenochrome at 480 nm. Extracts were prepared from Philodendron leaves and the edible portion and seeds of green peppers (Capsicum Annuum ). The tissues were minced, homogenized in 10 volumes of 16 mM Na-phosphate buffer pH 7.4 and centrifuged at 35, 000g for 30 min. The supernatant was dialyzed in 12,000 m.w. cut-off tubing, denatured in boiling water and centrifuged at 10, 000g for 10 min. Aliquots (5-50 ul) of the supernatant were assayed in 5 ml 16 mM Na-phosphate buffer pH 7.4 containing 100 uM ascorbate or norepinephrine. The denatured extracts had marked dose-dependent inhibitory effect on the autoxidation of ascorbic acid, with negligible influence on the formation of noradrenochrome . EDTA inhibited both reactions. The selectiveness of the extract toward the autoxidation of ascorbic acid makes it unlikely that the inhibitory effect is based on sequestering metal-ions.