Biomedical subjects

A H Datko

Publications and source records attributed to A H Datko.

At least 19 recordsLinked to original sources

PubMed1990

Phosphatidylcholine synthesis in the rat: the substrate for methylation and regulation by choline.

Two lines of evidence led us to reexamine the possibility that methylation of phosphoethanolamine and its partially methylated derivatives, in addition to methylation of the corresponding phosphatidyl derivatives, plays a role in mammalian phosphatidylcholine biosynthesis: (a) Results obtained by Salerno and Beeler with rat [Salerno, D.M. and Beeler, D.A. (1973) Biochim. Biophys. Acta 326, 325-338] appear to strongly support such a role for methylation of phosphobases; (b) Such reactions have recently been shown to play major roles in phosphatidylcholine synthesis by higher plants [see Datko, A.H. and Mudd, S.H. (1988) Plant Physiol. 88, 854-861 and references therein]. We found that, following continuous labeling of rat liver with L-[methyl-3H]methionine for 10.4 min (intraperitoneal administration) or for 0.75 min (intraportal administration), virtually no 3H was detected in methylated derivatives of phosphoethanolamine, but readily detectable amounts of 3H were present in the base moiety of each methylated derivative of phosphatidylethanolamine. Thus, there was no indication that phospho-base methylation makes a significant contribution. Studies of cultured rat hepatoma cells showed definitively for the first time in a mammalian system that choline deprivation up-regulates the rate of flow of methyl groups originating in methionine into phosphatidylethanolamine and derivatives. Even under these conditions, methylation of phosphoethanolamine bases appeared to play a negligible role.

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PubMed1990

The S-Methylmethionine Cycle in Lemna paucicostata.

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 S-methylmethionine 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.

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PubMed1989

Synthesis of methylated ethanolamine moieties: regulation by choline in lemna.

The results of experiments in which intact plants of Lemna paucicostata were labeled with either l-[(3)H(3)C]methionine, l-[(14)CH(3)]methionine, or [1,2-(14)C]ethanolamine support the conclusion that growth in concentrations of choline of 3.0 micromolar or above brings about marked decreases in the rate of biosynthesis of methylated forms of ethanolamine (normally present chiefly as phosphatidylcholine, with lesser amounts of choline and phosphocholine). The in vivo locus of the block is at the committing step in the biosynthetic sequence at which phosphoethanolamine is methylated by S-adenosylmethionine to form phosphomethylethanolamine. The block is highly specific: flow of methyl groups originating in methionine continues into S-adenosylmethionine, S-methylmethionine, the methyl moieties of pectin methyl ester, and other methylated metabolites. When choline uptake is less than the total that would be synthesized by control plants, phosphoethanolamine methylation is down-regulated to balance the uptake; total plant content of choline and its derivatives remains essentially constant. At maximum down-regulation, phosphoethanolamine methylation continues at 5 to 10% of normal. A specific decrease in the total available activity of AdoMet: phosphoethanolamine N-methyltransferase, as well as feedback inhibition of this enzyme by phosphocholine, and prevention of accumulation of phosphoethanolamine by down-regulation of ethanolamine synthesis may each contribute to effective control of phosphoethanolamine methylation. This down-regulation may necessitate major changes in S-adenosylmethionine metabolism. Such changes are discussed.

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PubMed1989

Synthesis of methylated ethanolamine moieties: regulation by choline in soybean and carrot.

Cultured cell suspensions of both carrot (Daucus carota L.) and soybean (Glycine max) take up exogenous choline efficiently from their respective growth media. During sustained growth at a concentration near 50 micromolar choline, this compound was taken up at rates which exceeded those at which phosphatidylcholine, is synthesized by cells growing in standard (i.e. choline-free) media. In 50 micromolar choline, both types of cells metabolized this compound to phosphocholine and phosphatidylcholine, but not to other detected metabolites, and marked accumulations of phosphocholine and choline occurred relative to phosphatidylcholine. Pregrowth in 50 micromolar choline for several doublings decreased the rate at which carrot cells transferred (3)H from l-[(3)H(3)C] methionine into the network of all methylated derivatives of ethanolamine by some 98%. With soybean cells, a decrease of 77% was observed. In both cell types, transfer of (3)H into S-methylmethionine, pectin methyl esters, methylated nucleic acids, and nonpolar lipid continued unabated. Gel-filtered extracts of carrot cells pregrown in 50 micromolar choline had marked decreases in the specific activities of S-adenosylmethionine-dependent phosphoethanolamine, phosphomethylethanolamine, and phosphodimethylethanolamine N-methyltransferases; extracts of soybean cells had a similar decrease in phosphoethanolamine N-methyltransferase. The significance of these findings for regulation of the rate of synthesis of methylated ethanolamine moieties is discussed.

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PubMed1989

Aspartokinase of Lemna paucicostata Hegelm. 6746.

A sensitive and specific method was developed for assay of aspartokinase (EC 2.7.2.4) in crude extracts of Lemna paucicostata. Lysine inhibited approximately 93%, and threonine approximately 6%; together, these amino acids inhibited 99%. Inhibition by lysine was synergistically increased by S-adenosylmethionine, which by itself had no effect on activity. Essentially complete inhibition of threonine-resistant activity was obtained with lysine, and of lysine-resistant activity with threonine. Inhibition by lysine and threonine was additive, with no indication of concerted inhibition. Aspartate concentration had no effect on the relative proportions of lysine- and threonine-sensitive activities. Aspartokinase activity was in large excess of that reported by other workers, the maximum capacity (V(max)) far exceeding the in vivo requirements. Estimations of rates of aspartokinase in vivo suggest that the step catalyzed by this enzyme may not be the overall ;rate-limiting' one for entry of 4-carbon units into the aspartate family of amino acids, and that feedback inhibition of this enzyme by lysine and threonine may not be a major factor in regulating flux through this step.

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PubMed1989

Regulatory Structure of the Biosynthetic Pathway for the Aspartate Family of Amino Acids in Lemna paucicostata Hegelm. 6746, with Special Reference to the Role of Aspartokinase.

Comprehensive studies were made with Lemna paucicostata Hegelm. 6746 of the effects of combinations of lysine, methionine, and threonine on growth rates, soluble amino acid contents, aspartokinase activities, and fluxes of 4-carbon moieties from aspartate through the aspartokinase step into the amino acids of the aspartate family. These studies show that flux in vitro through the aspartokinase step is insensitive to inhibition by lysine or threonine, and confirm previous in vitro data in establishing that aspartokinase in vivo is present in two orders of magnitude excess of its requirements. No evidence of channeling of the products of the lysine- and threonine-sensitive aspartokinases was obtained, either form of the enzyme alone being more than adequate for the combined in vivo flux through the aspartokinase step. The marked insensitivity of flux through the aspartokinase step to inhibition by lysine or threonine strongly suggests that inhibition of aspartokinase by these amino acids is not normally a major factor in regulation of entry of 4-carbon units into the aspartate family of amino acids. Direct measurement of fluxes of 4-carbon units demonstrated that: (a) Lysine strongly feedback regulates its own synthesis, probably at the step catalyzed by dihydrodipicolinate synthase. (b) Threonine alone does not regulate its own synthesis in vivo, thereby confirming previous studies of the metabolism of [(14)C]threonine and [(14)C]homoserine in Lemna. This finding excludes not only aspartokinases as an important regulatory determinant of threonine synthesis, but also two other enzymes (homoserine dehydrogenase and threonine synthase) suggested to fulfill this role. Complete inhibition of threonine synthesis was observed only in the combined presence of accumulated threonine and lysine. The physiological significance of this single example of apparent regulation of flux at the aspartokinase step, albeit under unusually stringent conditions of aspartokinase inhibition, remains to be determined. (c) Isoleucine strongly inhibits its own synthesis, probably at threonine dehydratase, without causing compensatory reduction in threonine synthesis. A fundamentally changed scheme for regulation of synthesis of the aspartate family of amino acids is presented that has important implications for improvement of the nutritional contents of these amino acids in plants.

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PubMed1989

Synthesis of Ethanolamine and Its Regulation in Lemna paucicostata.

The metabolism of ethanolamine and its derivatives in Lemna paucicostata has been investigated, with emphasis on the path-way 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: serine --> ethanolamine --> phosphoethanolamine --> phosphatidylethanolamine. A possible alternative is: serine --> phosphatidylserine --> phosphatidylethanolamine --> ethanolamine --> phosphoethanolamine. Cell-free extracts of L. paucicostata were shown to produce CO(2) 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.

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PubMed1988

In Vivo Regulation of Threonine and Isoleucine Biosynthesis in Lemna paucicostata Hegelm. 6746.

Little, if any, regulation of threonine synthesis was observed in Lemna paucicostata Hegelm. 6746 supplemented with concentrations of threonine and/or isoleucine that allow for uptake of these amino acids in amounts sufficient for total plant requirements, and that increase tissue concentrations of soluble threonine manyfold. High tissue concentrations of soluble threonine generated endogenously in isoleucine-supplemented plants were no more effective in regulation than a similar concentration of threonine accumulated from the medium. These studies exclude also major regulation of threonine biosynthesis by bivalent repression by threonine plus isoleucine. Isoleucine biosynthesis was severely inhibited by supplementation with isoleucine, but not with threonine or methionine. The fivefold increase in soluble threonine in isoleucine-supplemented plants suggests that threonine dehydratase is a major locus for feedback regulation of isoleucine synthesis. It is concluded that regulation of threonine biosynthesis differs from that of the other amino acids of the aspartate family (isoleucine, methionine, and lysine), each of which strongly feedback regulates its own synthesis. Methionine supplementation had a negligible effect on the tissue concentration of soluble threonine, indicating that threonine is not important in balancing changes of flux into methionine by equivalent changes of flux through the step catalyzed by aspartokinase.

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PubMed1988

Phosphatidylcholine synthesis: differing patterns in soybean and carrot.

The methylation steps in the biosynthesis of phosphatidylcholine by tissue culture preparations of carrot (Daucus carota L.) and soybean (Glycine max), and by soybean leaf discs, have been studied. Preparations were incubated with tracer concentrations of l-[(3)H(3)C]methionine and the kinetics of appearance of radioactivity in phosphomethylethanolamine, phosphodimethylethanolamine, phosphocholine, phosphatidylmethylethanolamine, phosphatidyldimethylethanolamine, phosphatidylcholine, methylethanolamine, dimethylethanolamine, and choline followed at short incubation times. With soybean (tissue culture or leaves), an initial methylation utilizes phosphoethanolamine as substrate, forming phosphomethylethanolamine. The latter is converted to phosphatidylmethylethanolamine, which is successively methylated to phosphatidyldimethyethanolamine and to phosphatidylcholine. With carrot, again, an initial methylation is of phosphoethanolamine. Subsequent methylations occur at both the phospho-base and phosphatidyl-base levels. Both of these patterns differ qualitatively from that previously demonstrated in Lemna (SH Mudd, AH Datko 1986 Plant Physiol 82: 126-135) in which all three methylations occur at the phospho-base level. For soybean and carrot, some added contribution from initial methylation of phosphatidylethanolamine has not been excluded. These results, together with those from similar experiments carried out with water-stressed barley leaves (WD Hitz, D Rhodes, AD Hanson 1981 Plant Physiol 68: 814-822) and salinized sugarbeet leaves (AD Hanson, D Rhodes 1983 Plant Physiol 71: 692-700) suggest that in higher plants some, perhaps all, phosphatidylcholine synthesis occurs via a common committing step (conversion of phosphoethanolamine to phosphomethylethanolamine) followed by a methylation pattern which differs from plant to plant.

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PubMed1988

Enzymes of phosphatidylcholine synthesis in lemna, soybean, and carrot.

Cell-free extracts from Lemna and suspension cultured carrot (Daucus Carota L.) catalyze S-adenosylmethionine-dependent N-methylations of phosphoethanolamine, phosphomethylethanolamine, and phosphodimethylethanolamine; extracts of suspension cultured soybean (Glycine max), of phosphoethanolamine only. Material pelleted from each tissue between 15,000 and 100,000g catalyzes S-adenosylmethionine-dependent N-methylations of phosphatidylmethylethanolamine and phosphatidyl-dimethylethanolamine, but not phosphatidylethanolamine. Extracts from each tissue catalyze CTP-dependent cytidylyltransfers to each of the three methylated phosphoethanolamine derivatives, forming the corresponding CDP derivatives. Some of the properties of the activities investigated are reported. On the basis of in vivo labeling experiments, we have proposed (AH Datko, SH Mudd 1988 Plant Physiol 88: 854-861) differing pathways for phosphatidylcholine synthesis in which, after a common committing step, N-methylation of phosphoethanolamine, subsequent methylations occur in Lemna almost exclusively at the phospho-base level; in soybean, at the phosphatidyl-base level; and in carrot, at both levels. Thus, among the activities investigated, at least those required for the operation of the proposed pathways have been positively demonstrated. The extent to which the present results explain the differences between these pathways is discussed, and a speculation offered as to how these differences may have arisen phylogenetically.

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PubMed1986

Methionine methyl group metabolism in lemna.

To provide information upon the ways in which Lemna paucicostata uses the methyl group of methionine, plants were grown for various periods (from 1 minute to 6.8 days) in the presence of a tracer dose of radioactive methyl-labeled methionine. Protein methionine accounted for approximately 19% of the accumulated methyl moieties; other methylated products, about 81%. The latter group included (percent of total methyl in parentheses): methylated ethanolamine derivatives (46%); methyl esters of the pellet (chiefly, or solely, pectin methyl esters) (15%); chlorophyll methyl esters (8%); unidentified neutral lipids (6%); nucleic acid derivatives (2-5%); methylated basic amino acids (2%). No other major methylated compounds were observed in any plant fraction. Available evidence suggests that little, if any, oxidation of the methyl group of methionine, directly or indirectly, occurs in Lemna. Our results indicate that S-methyl-methionine sulfonium is formed relatively rapidly, but does not accumulate at a commensurate rate, probably being reconverted to methionine. To our knowledge, this is the first time a reasonably complete accounting of the metabolic fate of methionine methyl has been obtained for any plant. The extent to which the results with Lemna may be representative of the situation for other higher plants is discussed.

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PubMed1986

Uptake of Choline and Ethanolamine by Lemna paucicostata Hegelm. 6746.

Lemna paucicostata Hegelm. 6746 possesses specific systems for uptake of choline and ethanolamine. Each is distinct from the six other systems for uptake of organic compounds so far identified in this plant. Both systems show biphasic kinetics, so that uptake by them can be described as the composite result of two Michaelis-Menten processes. Inhibitor studies are reported which indicate the very strict structural specificity of each system. The kinetic constants of choline uptake are such that, at an external concentration of 0.65 micromolar, the total requirement of the plant for this compound would be met, 41% via the high affinity system and 59% via the lower. At an external concentration of 2.4 micromolar ethanolamine, an amount of this compound sufficient to form the total choline of the plant would be supplied, 59% via the high affinity system and 41% via the lower. These, and other observations, strongly support the physiological importance of these systems under natural conditions.

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PubMed1986

Effects of Orthophosphate and Adenosine 5'-Phosphate on Threonine Synthase and Cystathionine gamma-Synthate of Lemna paucicostata Hegelm. 6746.

Inorganic phosphate (Pi) inhibits threonine synthase of Lemna, and cystathionine gamma-synthase less strongly. AMP is an extremely potent and structurally specific inhibitor of threonine synthase. Each inhibition progressively decreases with increasing concentrations of O-phosphohomoserine (OPH). To study the in vivo effects of these inhibitions, Lemna was grown with a range of Pi concentrations. A 25,000-fold increase in Pi concentration in the culture medium caused an increase of only 6-fold in total phosphorus of the plants. This is explained by the fact that a high affinity Pi uptake system is selectively down-regulated during growth with high concentrations of Pi. Pi and AMP in plants grown with various Pi concentrations were determined, and concentrations estimated for chloroplasts, the organelle containing threonine synthase and cystathionine gamma-synthase. Calculations indicated that for growth at standard external Pi (0.4 millimolar) or above, if total OPH were uniformly distributed within the plants, activities of the two enzymes in question would be severely inhibited, and each would fall two orders of magnitude below the amount required to provide threonine (plus isoleucine) or methionine adequate for growth. If OPH were restricted to chloroplasts, these inhibitions would be much less severe, resulting in enzyme activities approaching the required physiological amounts. Evidence is presented that even up to 50 millimolar external Pi, this ion does not limit production of threonine or methionine sufficiently to retard growth, consistent with the postulated localization of OPH within chloroplasts.

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PubMed1986

Phosphoethanolamine bases as intermediates in phosphatidylcholine synthesis by lemna.

The pathway for synthesis of phosphatidylcholine, the dominant methyl-containing end product formed by Lemna paucicostata, has been investigated. Methyl groups originating in methionine are rapidly utilized by intact plants to methylate phosphoethanolamine successively to the mono-, di-, and tri-methyl (i.e. phosphocholine) phosphoethanolamine derivatives. With continued labeling, radioactivity initially builds up in these compounds, then passes on, accumulating chiefly in phosphatidylcholine (34% of the total radioactivity taken up by plants labeled to isotopic equilibrium with l-[(14)CH(3)]methionine), and in lesser amounts in soluble choline (6%). Radioactivity was detected in mono- and dimethyl derivatives of free ethanolamine or phosphatidylethanolamine only in trace amounts. Pulse-chase experiments with [(14)CH(3)]choline and [(3)H] ethanolamine confirmed that phosphoethanolamine is rapidly methylated and that phosphocholine is converted to phosphatidylcholine. Initial rates indicate that methylation of phosphoethanolamine predominates over methylation of either phosphatidylethanolamine or free ethanolamine at least 99:1. Although more studies are needed, it is suggested this pathway may well turn out to account for most phosphatidylcholine synthesis in higher plants. Phosphomethylethanolamine and phosphodimethylethanolamine are present in low quantities during steady-state growth (18% and 6%, respectively, of the amount of phosphocholine). Radioactivity was not detected in CDP-choline, probably due to the low steady-state concentration of this nucleotide.

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PubMed1985

In vivo regulation of de novo methionine biosynthesis in a higher plant (lemna).

Administration of methionine to growing Lemna had essentially no effect on accumulation of sulfate sulfur in protein cysteine, but decreased accumulation into cystathionine and its products (homocysteine, methionine, S-methylmethioninesulfonium salt, S-adenosylmethionine, and S-adenosylhomocysteine) to as low as 21% that of control plants, suggesting that methionine regulates its own de novo synthesis at cystathionine synthesis. Methionine caused only a slight reduction (to 80% that of control plants) in the accumulation of sucrose carbon into the 4-carbon moieties of cystathionine and products. This observation was puzzling since cystathionine synthesis proceeds by incorporation of equivalent amounts of sulfur (from cysteine) and 4-carbon moieties (from O-phosphohomoserine). The apparent inconsistency was resolved by the demonstration in Lemna (Giovanelli, Datko, Mudd, Thompson 1983 Plant Physiol 71: 319-326) that de novo synthesis of the methionine 4-carbon moiety occurs not only via the established transsulfuration route from O-phosphohomoserine, but also via the ribose moiety of 5'-methylthioadenosine. It is now clear that the more accurate assessment of the flux of sulfur (and 4-carbon moieties) through transsulfuration is provided by the amount of (35)S from (35)SO(4) (2-) that accumulates in cystathionine and its products, rather than by the corresponding measurements with (14)C. These studies therefore unequivocally demonstrate in higher plants that methionine does indeed feedback regulate it own de novo synthesis in vivo, and that cystathionine synthesis is a locus for this regulation.

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PubMed1985

Uptake of Amino Acids and Other Organic Compounds by Lemna paucicostata Hegelm. 6746.

A survey of the capacity of Lemna paucicostata to take up organic compounds such as might be present in the natural environment of this plant has identified eight discrete transport systems. Reciprocal inhibition studies defined the preferred substrates for these systems as follows: (a) neutral l-alpha-amino acids, (b) basic amino acids, (c) purine bases, (d) choline, (e) ethanolamine, (f) tyramine, (g) urea, and (h) aldohexoses. Each of these systems takes up its preferred substrates at high rates. At low concentrations, each Lemna frond during each minute takes up amounts which would be found in volumes ranging from 0.4 (tyramine) to 3.9 (urea) times its own volume. The two systems for amino acid transport both showed kinetics of the biphasic type, so that uptake by each can be described as the composite result of two Michaelis-Menten processes. The neutral amino acid system neither transports basic amino acids nor is inhibited by these compounds. The basic amino acid system does not transport neutral amino acids but is strongly inhibited by some, but not all, of these compounds. It is argued that the maintenance of these active, specific, and discrete systems in Lemna suggests they play important roles permitting this plant to utilize organic compounds occurring naturally in its environment.

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PubMed1985

Quantitative analysis of pathways of methionine metabolism and their regulation in lemna.

Individual rates of metabolism of the sulfur, methyl, and 4-carbon moieties of methionine were estimated in Lemna paucicostata Hegelm. 6746 growing under standard conditions, and used to quantitate pathways of methionine metabolism. Synthesis of S-adenosylmethionine (AdoMet) is the major pathway for methionine metabolism, with over 4 times as much methionine metabolized by this route as accumulates in protein. More than 90% of AdoMet is used for transmethylation. Methyl groups of choline, phosphatidylcholine, and phosphorylcholine are major end products of this pathway. Flux through methylthio recycling is about one-third the amount of methionine accumulating in protein. Spermidine synthesis accounts for at least 60% of the flux through methylthio recycling. The results obtained here, together with those reported for methionine-supplemented plants (Giovanelli, Mudd, Datko 1981 Biochem Biophys Res Commun 100: 831-839), indicate that methionine supplementation reduced methylneogenesis by no more than the small amount expected from the reduced entry of sulfate sulfur into methionine (Giovanelli, Mudd, Datko, 1985 Plant Physiol 77: 450-455). Methionine supplementation had no significant effect on transmethylation or methylthio recycling. The combined data provide the first comprehensive estimates of the quantitative relationships of major pathways for methionine metabolism and their control in plants.

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PubMed1984

Sulfate Uptake and Its Regulation in Lemna paucicostata Hegelm. 6746.

The results of studies of SO(4) (2-) uptake by Lemna paucicostata are most simply interpreted by the hypothesis that at least two components are involved, one saturating and one linear, ;nonsaturating.' The saturating component has a low K(m) and high specificity for SO(4) (2-). Uptake by the nonsaturating component is less affected by pH and temperature than is that of the saturating system. SO(4) (2-) efflux is not quantitatively important in Lemna under standard conditions (20 micromolar SO(4) (2-)) (Datko AH, SH Mudd 1980 Plant Physiol 65: 906-912). 55% of newly taken up (35)SO(4) (2-) enters a slowly turning over compartment (vacuole?); 45% remains in a compartment (cytoplasm?) in which it is rapidly metabolized to organic compounds.Growth in increased concentrations of SO(4) (2-) or cystine, but not methionine, down-regulates the saturating, but not the nonsaturating, system. Growth in limiting SO(4) (2-) up-regulates the saturating system. Overall, a 500-fold change was observed. Reciprocal inhibition experiments demonstrated that molybdate and SO(4) (2-) are taken up by a common mechanism, but growth in molybdate failed to up-regulate SO(4) (2-) uptake. Regulation by growth in SO(4) (2-) or cystine did not markedly affect uptake of phosphate or of several organic compounds.The saturating system contributes 99% of SO(4) (2-) uptake under standard conditions, providing sufficient SO(4) (2-) so it is not limiting. In nature the same system likely contributes at least 65 to 70%.

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