1-Aminocyclopropane-l-carboxylic Acid - An Intermediate of the Ethylene Biosynthesis in Higher Plants

Abstract

Ethylene biosynthesis in plant tissues is enhanced by addition of 1-aminocyclopropane-l-carboxylic acid (ACPCA) up to 100 times depending on the concentration. Like the endogenous ethylene biosynthesis ACPCA-dependent ethylene production is inhibited completely under anaerobic conditions. Structural integrity of the plant tissue is essential for the endogenous as well as the ACPCA-dependent ethylene biosynthesis; osmotic shock inhibits both of them. C0>Cl2, reported to inhibit ethylene biosynthesis, also inhibits ACPCA-dependent ethylene production. On the contrary, L-2-amino-4-(2-aminoethoxy)-trans-3butenoic acid (RO 4468), a potent inhibitor of the conversion of methionine to ethylene and 5-methylthioribose with S-adenosylmethionine as an intermediate, does not inhibit the ACPCA-dependent ethylene biosynthesis. High concentrations of methionine slightly stimulate ethylene production in our system but the rate of ACPCA-dependent ethylene biosynthesis is strongly inhibited. We interprete the data as follows: methionine is converted to S-adenosylmethionine as known from the literature. S-adenosylmethionine decomposes to 5'-methylthioadenosine and ACPCA. As a key intermediate the Schiff-base of ACPCA and pyridoxalphosphate, activated for example by coenzyme A as a leaving group, is suggested. Possibly already S-adenosylmethionine is converted to the CoA-activated Schiff-base of pyridoxalphosphate with the implication that the formation of free ACPCA in vivo is only a side chain reaction. Upon hydride ion uptake, the activated ACPCA decomposes to ethylene and glycine or glyoxylic acid as end products. A detailed reaction scheme is proposed.