The S-Methylmethionine Cycle in Lemna paucicostata

Abstract

The metabolism of S-methylmethionine has been studied in cultures of plants of Lemna paucicostata and of cells of carrot (Daucus carota) and soybean (Glycine max). In each system, radiolabeled S-methylmethionine was rapidly formed from labeled L-methionine, consistent with the action of S-adenosyl-L-methionine:methionine S-methyltransferase, an enzyme which was demonstrated during these studies in Lemna homogenates. In Lemna plants and carrot cells radiolabel disappeared rapidly from Smethylmethionine during chase incubations in nonradioactive media. The results of pulse-chase experiments with Lemna strongly suggest that administered radiolabeled S-methylmethionine is metabolized initially to soluble methionine, then to the variety of compounds formed from soluble methionine. An enzyme catalyzing the transfer of a methyl group from S-methylmethionine to homocysteine to form methionine was demonstrated in homogenates of Lemna. The net result of these reactions, together with the hydrolysis of S-adenosylhomocysteine to homocysteine and adenosine, is to convert S-adenosylmethionine to methionine and adenosine. A physiological advantage is postulated for this sequence in that it provides the plant with a means of sustaining the pool of soluble methionine even when overshoot occurs in the conversion of soluble methionine to S-adenosylmethionine. The facts that the pool of soluble methionine is normally very small relative to the flux into S-adenosylmethionine and that the demand for the latter compound may change very markedly under different growth conditions make it plausible that such overshoot may occur unless the rate of synthesis of S-adenosylmethionine is regulated with exquisite precision. The metabolic cost of this apparent safeguard is the consumption of ATP. This S-methylmethionine cycle may well function in plants other than Lemna, but further substantiating evidence is neeeded. [6,25,26,35]). After administration of radiolabeled Met to a variety of intact plant preparations, substantial amounts of radiolabel have usually been found in SMM (4,18,19,21,23,29,34,37

Biosynthesis of 3-Dimethylsulfoniopropionate in Wollastonia biflora (L.) DC. (Evidence That S-Methylmethionine Is an Intermediate)

“…This gives new significance to SMM in plant metabolism. Although SMM is very common among higher plants, its only previously recognized fates were reconversion to Met via the SMM cycle and perhaps cleavage to homoserine and DMS (Giovanelli et al, 1980;Mudd and Datko, 1990). Our data implicate conversion of Met to SMM as a control point in DMSP biosynthesis; salinity almost certainly promotes this conversion because the increase in DMSP leve1 upon salinization (some 15 pmol g-' fresh weight) is 50-fold larger than the SMM pool in unsalinized leaves.…”

“…Taken with the kinetic data in Figure 4, The results presented so far make a good prima facie case that SMM is an intermediate in DMSP biosynthesis. However, the interpretation of radiolabeling experiments with Met and SMM is potentially complicated by the interconversion of these compounds via the SMM cycle (Mudd and Datko, 1990). In the first half of this (futile) cycle, Met is methylated by S-adenosylmethionine to SMM; in the second half, SMM combines with homocysteine to give two molecules of Met.…”

“…SMM labeled with two methyl-'H3 groups was supplied together (Giovanelli et al, 1989). However, such an effect seems unlikely to have shut down the SMM cycle in these experiments, inasmuch as the cycle would itself furnish the homocysteine needed for its operation (Mudd and Datko, 1990).…”

“…Because W. biflora is widely distributed and highly variable, we examined DMSP accumulation in genotypes from diverse geographic origins before selecting plants and conditions for experiments. We (Mudd and Datko, 1990), and of Met and…”

The N-Terminal Region of Arabidopsis Cystathionine gamma -Synthase Plays an Important Regulatory Role in Methionine Metabolism

“…SAM is also the precursor of plant metabolites such as ethylene, polyamines, vitamin B1, and the Fe-chelator mugineic acid (Anderson, 1990;Ma et al, 1995;Sun, 1998). In addition, Met also serves as a donor for secondary metabolites through S-methyl-Met (SMM; Mudd and Datko, 1990). As can be expected of its cellular importance, Met biosynthesis is subject to complex regulatory control whose mechanism is only now being gradually clarified.…”