Abstract
After harvest, broccoli (Brassica oleracea L. var. italica) florets senesce rapidly at ambient temperatures, seen in rapid yellowing (degreening) and loss of chlorophyll and ascorbic acid. During senescence the rate of ethylene production significantly increased. ACC (1-aminocyclopropane- 1-carboxylic acid) oxidase activity in florets greatly increased, reaching a peak and then declining sharply, paralleling the pattern of ethylene production. Increase in ACC oxidase activity was enhanced by supplying exogenous ethylene, and suppressed by treatment with 2,5-norbornadiene (NBD). NBD also retarded chlorophyll loss and thus suppressed the rapid yellowing of florets. The results indicate that endogenous ethylene may play a role in senescence of broccoli florets, and may regulate the induction of ACC oxidase activity. ACC oxidase was extracted and partially purified from broccoli florets. Activity required Fe 2+ , ascorbate and CO 2 (HCO 3 ) as cofactors in addition to ACC and oxygen as substrates. The Km value of the enzyme for ACC was found to be 55.6 μM.
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