Lipid peroxidation forms ethylene from 1-aminocyclopropane-1-carboxylic acid and may operate in leaf senescence.
An enzyme system is described which oxidizes 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene under physiological conditions. It comprises linoleic acid, pyridoxal phosphate, manganese, and lipoxygenase (linoleate:oxygen oxidoreductase, EC 1.13.11.12). It requires oxygen and is specific for manganese; it can operate but only with greatly reduced yield in the absence of pyridoxal phosphate. An enzyme with the same properties was prepared from microsomal membranes of the seedling shoots of peas. Both have similar reactions to a variety of inhibitors and other reagents. The properties also resemble those of at least two of the in vivo systems recorded in the literature. Intact green oat leaves also contain a similar system. Because there is a growing body of evidence that ethylene formation is associated with cell membranes and because the yields of ethylene from the complete system are much higher than those recorded for other enzymes, it may be identical with the in vivo system acting in senescent leaves.
Thimann Laboratories, University of California, Santa Cruz, CA 95064.
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Thimann