Synthesis of Ethanolamine and Its Regulation in Lemna paucicostata

Abstract

The metabolism of ethanolamine and its derivatives in Lemna paucicostata has been investigated, with emphasis on the pathway for synthesis of phosphoethanolamine, a precursor of phosphatidylcholine in higher plants. In experiments involving labeling of intact plants with radioactive serine, ambiguities of interpretation due to entry of radioactivity into methyl groups of methylated ethanolamine derivatives were mitigated by pregrowth of plants with methionine. Difficulties due to labeling of diacylglyceryl moieties of phospholipids were avoided by acid hydrolysis of crucial samples and determination of radioactivity in isolated serine or ethanolamine moieties. The results obtained from such experiments are most readily reconciled with the biosynthetic sequence: seine --ethanolamine --phosphoethanolamine phosphatidylethanolamine. A possible altemative is: serine phosphatidylserine -. phosphatidylethanolamine -i ethanolamine --phosphoethanolamine. Cell-free extracts of L. paucicostata were shown to produce CO2 from the carbon originating as C-1 of serine at a rate sufficient to satisfy the demand for ethanolamine moieties. A number of experiments produced no support for a hypothetical role for phosphoserine in phosphoethanolamine formation. Uptake of exogenous ethanolamine commensurately down-regulates the synthesis of ethanolamine moieties (considered as a whole, and regardless of their state of derivatization at the time of their formation). In agreement with previous observations, uptake of exogenous choline down-regulates the methylation of phosphoethanolamine, without being accompanied by secondary accumulation of a marked excess of ethanolamine derivatives.findings bring to the fore the question of the biosynthetic origin in higher plants of P-EA, a compound crucial to initiation of each of these metabolic pathways. One route to P-EA, each of the component steps of which has been demonstrated in higher plants, involves the decarboxylation of PtdSerine:PtdSerine --PtdEA + CO2 (reaction 1)PtdSerine decarboxylase activity has been demonstrated in extracts of spinach leaves, chiefly in the 100,000g supernatant fraction (26,27 Plant Physiol. Vol. 91, 1989 10, 18), contribution to EA formation. EA is a widespread constituent of higher plants (23), but a decarboxylase unequivocally acting on free serine (5, 10) has not, to our knowledge, been reported.Once EA is formed, the pathway to P-EA may be completed by EA kinase:This kinase is well known in higher plants (29,30), including Lemna (32,33).An alternative route to P-EA, entirely speculative at the moment (32), would involve the decarboxylation of P-serine:Evidence (reviewed and summarized by Keys [21 ]) is available suggesting that P-serine may be formed in at least some higher plants from the photosynthetic and glycolytic intermediate, P-glycerate, by successive oxidation and transamination.An additional uncertainty related to P-EA metabolism in higher plants is whether this compound can be converted to PtdEA by a sequence analogous to that for the c...

Enhanced synthesis of choline and glycine betaine in transgenic tobacco plants that overexpress phosphoethanolamine N -methyltransferase

“…EA synthesis in plants is thought to proceed mainly by decarboxylation of free serine (33), not phosphatidylserine as in other organisms (2). Because serine decarboxylases were recently cloned from plants (ref.…”

Plants Synthesize Ethanolamine by Direct Decarboxylation of Serine Using a Pyridoxal Phosphate Enzyme

“…The 14 C]serine and isolated by chromatography on a 3-ml column of AG-1 (formate) (1). Final purification was by thin layer electrophoresis for 20 min on cellulose plates at 4°C, using buffer containing 2.5% pyridine and 2.5% glacial acetic acid (v/v).…”

“…1 is known to be derived from serine, but the biochemical mechanism has not been established (1,2). There are several possibilities (Fig.…”

“…Another potential mechanism for Etn synthesis is direct decarboxylation of free serine, and the pattern of radiolabeled serine metabolism in the aquatic plant Lemna is most readily reconciled with this possibility (1). Also apparently supporting direct decarboxylation are reports that extracts of Lemna fronds (1) and cucurbit seedlings (12) contain serine decarboxylase (SDC) activity.…”

Choline Synthesis in Spinach in Relation to Salt Stress

“…The point of regulation was tentatively identified by Mudd and Datko (1989~) as the point of the first N-methyltransfer, i.e. the enzyme converting PEA to PMEA.…”