Serine Biosynthesis in Rat Liver

Enzymes and the assessment of protein nutrition

This symposium is concerned with the assessment of protein nutritional status in man. The question we have to examine is whether measurements of enzyme activity are, or may be in the future, of any value for this purpose. There are several concepts which may be useful in considering this problem, such as the nature of enzymes and the regulation of their amount in the body, the effect on activity of a biochemical lesion at the cellular level, and the adaptive changes that reflect an altered balance of metabolic pathways.

The effects of a low protein diet on amino acids and enzymes in the serine synthesis pathway in mice

L-serine is required for cellular and tissue growth and is particularly important in the immature brain where it acts as a crucial neurotrophic factor. In this study, the levels of amino acids and enzymes in the L-serine biosynthetic pathway were examined in the forebrain, cerebellum, liver, and kidney after the exposure of mice to protein-restricted diets. The levels of L-serine, D-serine, and L-serine-O-phosphate were quantified by HPLC and quantitative Western blotting was used to measure changes in protein levels of five enzymes in the pathway. The L-serine biosynthetic enzyme phosphoserine phosphatase was strongly upregulated, while the serine degradative enzymes serine racemase and serine dehydratase were downregulated in the livers and kidneys of mice fed low (6%) or very low (2%) protein diets for 2 weeks compared with mice fed a normal diet (18% protein). No changes in these enzymes were seen in the brain. The levels of L-serine increased in the livers of mice fed 2% protein; in contrast, D-serine levels were reduced below the limit of detection in the livers of mice given either the 6 or 2% diets. D-Serine is a co-agonist at the NMDA class of glutamate receptors; no alterations in NMDA-R1 subunit expression were observed in liver or brain after protein restriction. These findings demonstrate that the expression of L-serine synthetic and degradative enzymes display reciprocal changes in the liver and kidney to increase L-serine and decrease D-serine levels under conditions of protein restriction, and that the brain is insulated from such changes.

Sulfur-Containing Amino Acid Metabolism in Parasitic Protozoa

Sulfur-containing amino acids play indispensable roles in a wide variety of biological activities including protein synthesis, methylation, and biosynthesis of polyamines and glutathione. Biosynthesis and catabolism of these amino acids need to be carefully regulated to achieve the requirement of the above-mentioned activities and also to eliminate toxicity attributable to the amino acids. Genome-wide analyses of enzymes involved in the metabolic pathways of sulfur-containing amino acids, including transsulfuration, sulfur assimilatory de novo cysteine biosynthesis, methionine cycle, and degradation, using genome databases available from a variety of parasitic protozoa, reveal remarkable diversity between protozoan parasites and their mammalian hosts. Thus, the sulfur-containing amino acid metabolic pathways are a rational target for the development of novel chemotherapeutic and prophylactic agents against diseases caused by protozoan parasites. These pathways also demonstrate notable heterogeneity among parasites, suggesting that the metabolism of sulfur-containing amino acids reflects the diversity of parasitism among parasite species, and probably influences their biology and pathophysiology such as virulence competence and stress defense.

Structural characterization of the reaction pathway in phosphoserine phosphatase: crystallographic "snapshots" of intermediate states

Phosphoserine phosphatase (PSP) is a member of a large class of enzymes that catalyze phosphoester hydrolysis using a phosphoaspartate-enzyme intermediate. PSP is a likely regulator of the steady-state d-serine level in the brain, which is a critical co-agonist of the N-methyl-d-aspartate type of glutamate receptors. Here, we present high-resolution (1.5-1.9 A) structures of PSP from Methanococcus jannaschii, which define the open state prior to substrate binding, the complex with phosphoserine substrate bound (with a D to N mutation in the active site), and the complex with AlF3, a transition-state analog for the phospho-transfer steps in the reaction. These structures, together with those described for the BeF3- complex (mimicking the phospho-enzyme) and the enzyme with phosphate product in the active site, provide a detailed structural picture of the full reaction cycle. The structure of the apo state indicates partial unfolding of the enzyme to allow substrate binding, with refolding in the presence of substrate to provide specificity. Interdomain and active-site conformational changes are identified. The structure with the transition state analog bound indicates a "tight" intermediate. A striking structure homology, with significant sequence conservation, among PSP, P-type ATPases and response regulators suggests that the knowledge of the PSP reaction mechanism from the structures determined will provide insights into the reaction mechanisms of the other enzymes in this family.

Phosphoserine phosphatase of human brain: partial purification, characterization, regional distribution, and effect of certain modulators including psychoactive drugs

Phosphoserine phosphatase (PSPase), a cytosolic enzyme has been purified 106 fold from human brain, by employing conventional protein purification techniques. The use of MgCl2 (10 mM) and chloroform treatment, during purification enabled the removal of non-specific proteins. The final enzyme preparation exhibited a broad pH optimum of 5.6-6.6 and could dephosphorylate both L and D enantiomers of the phosphoserine, but with different Km values for O-P-L serine (3.6 x 10(-5) M) and O-P-D serine (1 x 10(-4) M). Enzyme activity was found to be specific for phosphoserine, whereas other phosphoesters including phosphothreonine and phosphoproteins such as casein and phosvitin were found to be poor substrates. The enzyme activity was uncompetitively inhibited by L-serine. Further the PSPase activity was inhibited by vanadate, (41%), trifluoperazine (23%), chlorpromazine (34%) at an equimolar concentration of 1 mM, whereas lithium and ethanol did not influence the enzyme activity. Minor tranquilizers such as diazepam and chlordiazepoxide activated the enzyme activity to an extent of 13% and 59% respectively. In addition, species and regionwise heterogeneity was observed with respect to distribution of enzyme activity in six major areas of human, rabbit and rat brains.

Purification and properties of phosphoserine aminotransferase from bovine liver

L-Phosphoserine aminotransferase was purified from bovine liver to apparent homogeneity as judged by nondenaturing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, analytical ultracentrifugation, and immunochemical analysis. The purification procedure described involves the specific elution of the enzyme from Cibacron blue-agarose by micromolar concentrations of its substrate, phosphohydroxypyruvate. The purified enzyme had a specific activity of approximately 13 mumol of phosphohydroxypyruvate formed min-1 mg-1 of protein at 38 degrees C. Determinations of the native molecular weight and the subunit molecular weight indicated that the phosphoserine aminotransferase from bovine liver was a dimer composed of two subunits with identical molecular weights of 43,000.

Effects of high dietary methionine on activities of selected enzymes in the liver of kittens (Felis domesticus)

1. Growing male kittens were fed an 18% casein diet supplemented with 2, 3, or 4% L-methionine (MET) for 6 weeks. 2. Free MET concentration in liver increased 30-fold and cystathionine two- to three-fold; the activity of adenosyl-MET transferase and cystathionase also increased but remained lower than previously found in rats. 3. Taurine concentration in liver decreased in cats fed excess MET and appeared to depend on taurine intake. 4. Alanine aminotransferase activity was high in all groups while serine dehydratase activity was very low. 5. Pyruvate kinase and malic enzyme activities which are normally low in cat liver increased after excess MET. Also, glucose 6-phosphate and 6-phosphogluconate dehydrogenases increased. 6. Cat liver metabolism showed limited adaptation to an excess dietary intake of methionine compared to that found in rats.

The reactions of the phosphorylated pathway of L-serine biosynthesis: thermodynamic relationships in rat liver in vivo

The effect of the induction of the enzymes of the phosphorylated pathway of L-serine biosynthesis on the thermodynamic relationships among the reactions has been determined in rat liver in vivo. The mass action ratios of the reactions involved were calculated from the concentrations of appropriate metabolites in freeze-clamped liver from animals fed normal and low-protein diets for 2 weeks. These ratios were compared with the equilibrium constants of the same reactions previously determined under physiological conditions and the results previously obtained in the rabbit. The thermodynamic relationships in the pathway were different between the normal rat and rabbit as might have been expected, because of the significantly lower activities of the L-serine biosynthetic enzymes in the former animal. Although the delta G for the overall pathway is nearly identical in the rat and rabbit (-5.8 versus -5.5 kcal/mol, respectively), the distribution of delta G among the reactions is different. The disequilibrium in the pathway in rat liver is nearly equally divided between the L-phosphoserine phosphatase (EC 3.1.3.3) step and the other two reactions [D-3-phosphoglycerate dehydrogenase (EC 1.1.1.95) and L-phosphoserine aminotransferase (EC 2.6.1.52)], whereas in rabbit the phosphatase reaction accounts for nearly the entire delta G. Feeding the rat a low protein diet, however, induced the activity of D-3-phosphoglycerate dehydrogenase 12-fold, that of L-phosphoserine aminotransferase 20-fold, and that of L-phosphoserine phosphatase 2-fold. With the induction of the pathway, L-phosphoserine appeared in the tissue, there was a more than 3-fold rise in L-serine in the liver, and the pattern of delta G in the rat liver approached that in the rabbit.

Amino acid metabolism in euryhaline bivalves: regulation of glycine accumulation in ribbed mussel gills

Glycine levels in isolated ribbed mussel (Modiolus demissus) gill tissue increased slightly and decreased markedly when incubated at high and low salinities, respectively. Low levels of the enzymes involved in the biosynthesis of serine from triose phosphate intermediates, the serine hydroxymethyltransferase, and serine dehydrase were detected in gill tissue homogenates. Experiments using gill tissue incubated with (U-14C)-glycine and (U-14C)-serine indicated interconversion between serine and glycine and transfer of label to alanine, asparate, glutamate, CO2, organic acids, and protein. Glyoxylate was metabolized more slowly than glycine and was probably converted to glycine for catabolism. Studies using (1-14C)-glycine and (2-14C)-glycine with isolated gill tissue and mitochondria indicated that the mitochondrial glycine cleavage enzyme was the major route of glycine catabolism. Metabolic controls activating or inhibiting the glycine cleavage enzyme regulate tissue glycine accumulation and catabolism during hypersalinity or hyposalinity stress.

The reactions of the phosphorylated pathway of L-serine biosynthesis: thermodynamic relationships in rabbit liver in vivo

The thermodynamic relationships among the reactions of the phosphorylated pathway of L-serine biosynthesis have been determined in rabbit liver in vivo in different dietary states. The mass action ratios of the reactions involved were calculated from the concentrations of appropriate metabolites in freeze-clamped liver and compared with the equilibrium constants of the same reactions previously determined under physiological conditions. Toward this goal, a new, highly specific enzymatic assay for L-phosphoserine was developed to allow the accurate measurement of this intermediate in biological material. The level of L-phosphoserine, the immediate precursor of L-serine, varied significantly with diet, being 0.81, 0.38, and 0.21 mumol/g wet wt in the fed, and 24 h and 48 h fasted states, respectively. The tissue content of L-phosphoserine was also sensitive to anoxia, falling almost fivefold within 5 min after the liver was removed. Values of for the combined reactions of the first two steps of the pathway of L-serine biosynthesis [D-3-phosphoglycerate dehydrogenase (EC 1.1.1.95) and L-phosphoserine aminotransferase (EC 2.6.1.52)] in livers from animals in different dietary states were calculated to be 1.2 X 10(-4) (fed), 1.4 X 10(-4) (24 h starved), and 0.70 X 10(-4) (48 h starved), all being very close to the value of the combined equilibrium constant of the same reactions (2.44 X 10(-4). Even when there were major changes in the individual components of, such as a fivefold drop in L-phosphoserine and a sevenfold fall in alpha-ketoglutarate following 5 min of anoxia, remained relatively unchanged (2.7 X 10(-4). Thus, it has been concluded that, in rabbit liver under most normal conditions, the combined reactions of D-3-phosphoglycerate dehydrogenase and L-phosphoserine aminotransferase remain very near equilibrium, and that almost all of the disequilibrium of the pathway, amounting to a delta G of -5.5 kcal/mol in the fed state, is at the last step, the L-phosphoserine phosphatase reaction (EC 3.1.3.3).

Enzymes of serine metabolism in normal, developing and neoplastic rat tissues

The cellular pattern of serine metabolism was conceptualized into four main areas of metabolic sequences: the biosynthesis of serine from intermediates of the glycolytic pathway (the so-called "phosphorylated pathway"); and alternative pathways of serine utilization initiated by serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. The known regulatory and adaptive properties of the enzymes involved in these pathways were reviewed in detail and key enzymes associated with each pathway (phosphoserine aminotransferase, serine dehydratase, serine aminotransferase, and serine hydroxymethyltransferase, respectively) were selected for further investigation. Tissue distribution studies in the rat revealed that whereas serine dehydratase and serine aminotransferase activities were largely confined to the liver, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were more broadly distributed. In particular in tissues with a high rate of cell turnover, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were coordinately increased. An increase in serine hydroxymethyltransferase activity coincided temporally with the incorporation of [3-14C]serine and thymidine into DNA in normal human lymphocytes during proliferation after mitogenic stimulation by phytohemagglutinin. The evidence suggested a primarily gluconeogenic role for serine dehydratase and serine aminotransferase. Serine hydroxymethyltransferase has a role in providing glycine and one-carbon folate co-factors as precursors for nucleotide biosynthesis and in some situations serves to metabolically couple the pathway of serine biosynthesis to utilization for de novo purine and pyrimidine synthesis. Multiple enzymic forms were distinguished for serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. For serine dehydratase the two cytosolic multiple forms had no apparent functional significance; the multiple forms were catalytically unmodified by conditions promoting phosphorylation-dephosphorylation in vitro. The mitochondrial form of serine aminotransferase showed adaptive responses in gluconeogenic situations, and the hypothesis was proposed that the mitochondrial isoenzyme of serine hydroxymethyltransferase is associated together with serine aminotransferase in a pathway for gluconeogenesis from protein-derived amino acids such as glycine and hydroxyproline. The adaptive behaviour of the enzymes during the neonatal development of rat liver revealed that serine aminotransferase reached a peak in the mid-suckling period at a time when gluconeogenesis is known to be increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitrogen economy and the metabolism of serine and glycine in reticulocytes of rabbits

In this work it is demonstrated the NH3 potentially available from the oxidation of amino acids by the reticulocyte is utilized for the new-synthesis of serine via transamination reactions with hydroxypyruvate and phosphohydroxypyruvate. These compounds are derived from glucose, which furnishes in this manner the carbon skeleton of serine. It was shown that serine is mainly degraded via glycine, while glycine is in the main formed from serine. Furthermore it was found that serine synthesis is localized in the cytosol, while the reversible transformation of serine to glycine takes place in the mitochondria. A transfer of methylenetetrahydrofolate occurs in both directions across the mitochondrial membrane. A concentrative uptake of serine by the mitochondria was found, while glycine is transported slowly in both directions. On the basis of the data an overall balance is given of the quantitative relations between protein breakdown and hemoglobin synthesis as well as for the new-formation and utilization of serine and glycine. The new-formation of serine may amount to about one tenth of the glucose utilized by the reticulocyte and furnishes about one-half of the serine and glycine moieties required for the synthesis of heme and globin. The remainder is provided by the energy-dependent protein breakdown.

Asparagine metabolism in chicks and rats

Asparaginase and asparagine synthetase specific activity (enzyme units/mg protein) was determined in chicks and rats fed various protein diets. The specific activity of asparaginase was twofold higher in the kidney compared to the liver of chicks fed a 25% protein control diet, whereas the specific activity of asparaginase was eightfold greater in the liver compared to kidney tissue of rats fed the 25% protein control diet. The asparaginase specific activity in the chick liver, chick kidney, and rat liver were all significantly increased when animals were fed the 75% protein diet. Asparaginase specific activity increased significantly in rat and chick kidneys when the animals were fasted. Dietary supplementation of asparagine did not significantly increase asparaginase specific activity in either animal. The combined kidney and liver asparagine synthetase specific activity for chicks fed the control diets is over seven hundred times less than the corresponding combined asparaginase specific activity. Rats fed the control diet had a combined kidney and liver asparagine synthetase specific activity about 75 times less than the corresponding asparaginase specific activity. The asparagine synthetase specific activity was significantly increased in the rat and chick kidney when a 2% protein diet was fed.

Growth dependence of phosphoglyceric acid dehydrogenase activity in cultured rat liver cells

Rat liver epithelial cells in culture (WIRL-3C) have the enzymes that synthesize serine from 3-phophoglyceric acid. Both phosphoglyceric acid dehydrogenase (PGAD) and serine-phosphate (serine-P) forming activities fluctuate with time after subculture and are higher in growing than confluent cells. This activity pattern was not common for other dehydrogenases in WIRL-3C cells, nor was it common for PGAD activity in other cultured cells. At time of subculture, cells are removed from spent medium, treated with trypsin, and fed fresh medium. None of these parameters causes the rise in activity; in contrast, reduction in cell density and the accompanying stimulation of growth do. PGAD activity decreases when growth is slowed either as the cells progress to the end of the culture cycle, when cells are treated with dexamethasone-phosphate (Dx-P) or dibutyryl cyclic AMP(cAMP) and theophylline or when the serum concentration of the medium is reduced to 0.2%. Under these conditions, decreased PGAD activity is paralleled by a decline in growth and DNA accumulation. PGAD activity in WIRL-3C cells is regulated in a manner closely resembling what has been observed previously in rat liver from the whole animal. The possible use of this system in studying regulation of gene expression in mammalian cells is discussed.

Serine biosynthesis in human hair follicles by the phosphorylated pathway: follicular 3-phosphoglycerate dehydrogenase

The phosphorylated pathway of serine biosynthesis was demonstrated in human hair bulbs and sheaths by the formation of phosphoserine and serine from (14C)3-phosphoglyceric acid. The initial and rate limiting enzyme of the pathway, 3-phosphoglycerate dehydrogenase (3-PGDH) was demonstrated by enzyme determinations in human and rat hair follicles, human epidermis, and chicken epidermis. Follicular 3-PGDH was characterized using a sensitive fluorometric assay with NADH as a co-substrate. Monovalent cations (Na+, K+, Li+, or NH4+) were necessary for full enzyme activity. p-Hydroxymercuribenzoate inhibited activity, and activity was 3 times higher with NADH as a co-substrate than with NADPH. The apparent Km for the substrate hydroxyphosphopyruvic acid was 32.8 muM, and the apparent Km for NADH 4.8 muM similar to the Kms for other mammalian 3-PGDHs. Enzyme activity was not altered by parenteral corticosteroids, a high carbohydrate diet, low protein diet, or starvation. Enzyme activity decreased over the first 12 days of life in newborn rats. The phosphorylated pathway of serine synthesis provides a potential nondietary and nonhepatic source of serine, glycine, and their products in keratinizing tissues.

Effect of various concentrations of protein in chick diet upon the metabolic enzymes of glycine and serine

The effect of feeding low protein diets with and without 2% glycine or 2% L-serine to chicks upon the enzymes concerned in the matabolism of glycine and serine has been determined. D-3-phosphoglycerate dehydrogenase (EC 1.1.1.s), hosphoserine phosphatase (EC 3.1.3.3), and serine hydroxymethyltransferase (EC 2.1.2.1) activities were significantly increased in livers from chicks fed 75% protein diets as compared to liver enzyme activities from chicks fed either 24% chick starter, 2% or 25% protein diets. Phosphoserine phosphatase activity was significantly higher in kidney tissue of chicks fed 75% protein diets. Livers from chicks fed 25% protein diets had a higher activity for D-3-phosphoglycerate dehydrogenase and phosphoserine phosphatase than did those fed chick starter or 2% protein diets. D-3-phosphoglycerate dehydrogenase and phosphoserine phosphatase activities were higher in livers from chicks fed 2% protein, 2% protein + 2% glycine, and 2% protein + 2% L-serine diets when compared to those from chicks fed low protein diets with supplemental methionine or cysteine. Serine dehydratase (EC 4.2.1.13), glycerate dehydrogenase (EC 1.1.1.29) and hydroxypyruvate-P: L-glutamate transaminase activities remained constant in livers from chicks fed all experimental diets. The uric acid concentration was significantly increased in plasma from chicks fed the high protein diets which suggests that D-3-phosphoglycerate dehydrogenase, phosphoserine phosphatase and serine hydroxymethyltransferase activities were increased because of the high requirement for glycine in uric acid formation. The 75% protein diet provided three times as much glycine as the 25% protein diet which may have met the increased need for glycine for uric acid formation.

Distribution studies of glycine and serine enzymes in chicks

Day-old broiler type chicks were fed a practical starter ration for three weeks, sacrificed and the D-3-phosphoglycerate dehydrogenase (E.C.1.1.1.s), phosphoserine phosphatase (E.C.3.1.3.3.), glycerate dehydrogenase (E.C.1.1.1.29), hydroxypyruvate-P: glutamate transaminase, serine transhydroxymethylase (E.C.2.1.2.1), and serine dehydratase (E.C.4.2.1.13) enzyme activities were determined in liver, kidney, heart, spleen and brain tissue in evaluating glycine and serine metabolism. The distribution of the various glycine and serine biosynthetic enzymes in the chick tissues studied show that the major portion of glycine and serine synthesis occurs in hepatic tissue via the "phosphorylated pathway".

Enzymes and the assessment of protein nutriton

This symposium is concerned with the assessment of protein nutritional status in man. The question we have to examine is whether measurements of enzyme activity are, or may be in the future, of any value for this purpose. There are several concepts which may be useful in considering this problem, such as the nature of enzymes and the regulation of their amount in the body, the effect on activity of a biochemical lesion at the cellular level, and the adaptive changes that reflect an altered balance of metabolic pathways.