Phosphoserine phosphatase distribution in normal and neoplastic rat tissues

Role of Impaired Glycolysis in Perturbations of Amino Acid Metabolism in Diabetes Mellitus

The most frequent alterations in plasma amino acid concentrations in type 1 and type 2 diabetes are decreased L-serine and increased branched-chain amino acid (BCAA; valine, leucine, and isoleucine) levels. The likely cause of L-serine deficiency is decreased synthesis of 3-phosphoglycerate, the main endogenous precursor of L-serine, due to impaired glycolysis. The BCAA levels increase due to decreased supply of pyruvate and oxaloacetate from glycolysis, enhanced supply of NADH + H⁺ from beta-oxidation, and subsequent decrease in the flux through the citric acid cycle in muscles. These alterations decrease the supply of α-ketoglutarate for BCAA transamination and the activity of branched-chain keto acid dehydrogenase, the rate-limiting enzyme in BCAA catabolism. L-serine deficiency contributes to decreased synthesis of phospholipids and increased synthesis of deoxysphinganines, which play a role in diabetic neuropathy, impaired homocysteine disposal, and glycine deficiency. Enhanced BCAA levels contribute to increased levels of aromatic amino acids (phenylalanine, tyrosine, and tryptophan), insulin resistance, and accumulation of various metabolites, whose influence on diabetes progression is not clear. It is concluded that amino acid concentrations should be monitored in patients with diabetes, and systematic investigation is needed to examine the effects of L-serine and glycine supplementation on diabetes progression when these amino acids are decreased.

Serine Metabolism in Health and Disease and as a Conditionally Essential Amino Acid

L-serine plays an essential role in a broad range of cellular functions including protein synthesis, neurotransmission, and folate and methionine cycles and synthesis of sphingolipids, phospholipids, and sulphur containing amino acids. A hydroxyl side-chain of L-serine contributes to polarity of proteins, and serves as a primary site for binding a phosphate group to regulate protein function. D-serine, its D-isoform, has a unique role. Recent studies indicate increased requirements for L-serine and its potential therapeutic use in some diseases. L-serine deficiency is associated with impaired function of the nervous system, primarily due to abnormal metabolism of phospholipids and sphingolipids, particularly increased synthesis of deoxysphingolipids. Therapeutic benefits of L-serine have been reported in primary disorders of serine metabolism, diabetic neuropathy, hyperhomocysteinemia, and amyotrophic lateral sclerosis. Use of L-serine and its metabolic products, specifically D-serine and phosphatidylserine, has been investigated for the therapy of renal diseases, central nervous system injury, and in a wide range of neurological and psychiatric disorders. It is concluded that there are disorders in which humans cannot synthesize L-serine in sufficient quantities, that L-serine is effective in therapy of disorders associated with its deficiency, and that L-serine should be classified as a “conditionally essential” amino acid.

The PHGDH enigma: Do cancer cells only need serine or also a redox modulator?

Upregulation of serine biosynthesis pathway activity is an increasingly apparent feature of many cancers. Most notably, the first rate-limiting enzyme of the pathway, phosphoglycerate dehydrogenase (PHGDH), is genomically amplified in some melanomas and breast cancers and can be transcriptionally regulated by various tumor suppressors and oncogenes. Yet emerging evidence suggests that serine-in particular, serine biosynthetic pathway activity-may promote cancer in ways beyond providing the building blocks to support cell proliferation. Here, we summarize how mammalian cells tightly control serine synthesis before discussing alternate ways in which increased serine synthetic flux through PHGDH may benefit cancer cells, such as maintenance of TCA cycle flux through alpha-ketoglutarate (αKG) and modulation of cellular redox balance. We will also provide an overview of the current landscape of therapeutics targeting serine synthesis and offer a perspective on future strategies.

Overexpression of PSAT1 promotes metastasis of lung adenocarcinoma by suppressing the IRF1-IFNγ axis

An increasing number of enzymes involved in serine biosynthesis have been identified and correlated with malignant evolution in various types of cancer. Here we showed that the overexpression of phosphoserine aminotransferase 1 (PSAT1) is widely found in lung cancer tissues compared with nontumor tissues and predicts a poorer prognosis in patients with lung adenocarcinoma. PSAT1 expression was examined in a tissue microarray by immunohistochemistry. The data show that the knockdown of PSAT1 dramatically inhibits the in vitro and in vivo metastatic potential of highly metastatic lung cancer cells; conversely, the enforced expression of exogenous PSAT1 predominantly enhances the metastatic potential of lung cancer cells. Importantly, manipulating PSAT1 expression regulates the in vivo tumor metastatic abilities in lung cancer cells. Adjusting the glucose and glutamine concentrations did not alter the PSAT1-driven cell invasion properties, indicating that this process might not rely on the activation of its enzymatic function. RNA microarray analysis of transcriptional profiling from PSAT1 alternation in CL1-5 and CL1-0 cells demonstrated that interferon regulatory factor 1 (IRF1) acts as a crucial regulator of PSAT1-induced gene expression upon metastatic progression. Decreasing the IRF1-IFIH1 axis compromised the PSAT1-prompted transcriptional reprogramming in cancer cells. Our results identify PSAT1 as a key regulator by a novel PSAT1/IRF1 axis in lung cancer progression, which may serve as a potential biomarker and therapeutic target for the treatment of lung cancer patients.

Phosphoglycerate dehydrogenase is dispensable for breast tumor maintenance and growth

Cancer cells rely on aerobic glycolysis to maintain cell growth and proliferation via the Warburg effect. Phosphoglycerate dehydrogenase (PHDGH) catalyzes the first step of the serine biosynthetic pathway downstream of glycolysis, which is a metabolic gatekeeper both for macromolecular biosynthesis and serine-dependent DNA synthesis. Here, we report that PHDGH is overexpressed in many ER-negative human breast cancer cell lines. PHGDH knockdown in these cells leads to a reduction of serine synthesis and impairment of cancer cell proliferation. However, PHGDH knockdown does not affect tumor maintenance and growth in established breast cancer xenograft models, suggesting that PHGDH-dependent cancer cell growth may be context-dependent. Our findings suggest that other mechanisms or pathways may bypass exclusive dependence on PHGDH in established human breast cancer xenografts, indicating that PHGDH is dispensable for the growth and maintenance of tumors in vivo.

Functional genomics reveal that the serine synthesis pathway is essential in breast cancer

Cancer cells adapt their metabolic processes to drive macromolecular biosynthesis for rapid cell growth and proliferation (1,2). RNAi-based loss of function screening has proven powerful for the identification of novel and interesting cancer targets, and recent studies have used this technology in vivo to identify novel tumor suppressor genes (3). Here, we developed a method for identifying novel cancer targets via negative selection RNAi screening in solid tumours. Using this method, we screened a set of metabolic genes associated with aggressive breast cancer and stemness to identify those required for in vivo tumourigenesis. Among the genes identified, phosphoglycerate dehydrogenase (PHGDH) is in a genomic region of recurrent copy number gain in breast cancer and PHGDH protein levels are elevated in 70% of ER-negative breast cancers. PHGDH catalyzes the first step in the serine biosynthesis pathway, and breast cancer cells with high PHGDH expression have elevations in serine synthesis flux. Suppression of PHGDH in cell lines with elevated PHGDH expression, but not those without, causes a strong decrease in cell proliferation and a reduction in serine synthesis. We find that PHGDH suppression does not affect intracellular serine levels, but causes a drop in the levels of alpha-ketoglutarate, another output of the pathway and a TCA cycle intermediate. In cells with high PHGDH expression, the serine synthesis pathway contributes approximately 50% of the total anaplerotic flux of glutamine into the TCA cycle. These results reveal that certain breast cancers are dependent upon increased serine pathway flux caused by PHGDH over-expression and demonstrate the utility of in vivo negative selection RNAi screens for finding potential anticancer targets.

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.

Phosphoserine phosphatase deficiency in a patient with Williams syndrome

Decreased serine levels were found in plasma and cerebrospinal fluid (CSF) of a boy with pre- and postnatal growth retardation, moderate psychomotor retardation, and facial dysmorphism suggestive of Williams syndrome. Fluorescence in situ hybridisation with an elastin gene probe indicated the presence of a submicroscopic 7q11.23 deletion, confirming this diagnosis. Further investigation showed that the phosphoserine phosphatase (EC 3.1.3.3.) activity in lymphoblasts and fibroblasts amounted to about 25% of normal values. Oral serine normalised the plasma and CSF levels of this amino acid and seemed to have some clinical effect. These data suggest that the elastin gene and the phosphoserine phosphatase gene might be closely linked. This seems to be the first report of phosphoserine phosphatase deficiency.

3-Phosphoglycerate dehydrogenase deficiency: an inborn error of serine biosynthesis

Serine concentrations were markedly decreased in the cerebrospinal fluid of two brothers with congenital microcephaly, profound psychomotor retardation, hypertonia, epilepsy, growth retardation, and hypogonadism. The youngest boy also had congenital bilateral cataract. Magnetic resonance imaging of the brain showed evidence of dysmyelination. Plasma serine as well as plasma and cerebrospinal fluid glycine concentrations were also decreased but to a lesser extent. Treatment with oral serine in the youngest patient significantly increased cerebrospinal fluid serine and abolished the convulsions. In fibroblasts of both patients, a decreased activity was demonstrated of 3-phosphoglycerate dehydrogenase, the first step of serine biosynthesis (22% and 13% of the mean control value). This is an unusual disorder as the great majority of aminoacidopathies are catabolic defects. It is a severe but potentially treatable inborn error of metabolism that has not been previously reported in man.

Glucose contribution to nucleic acid base synthesis in proliferating hepatoma cells: a glycine-biosynthesis-mediated pathway

The coupling of glycolysis to serine and glycine metabolism was studied in fast-growing Zajdela hepatoma cultured cells. During the exponential phase of growth, occurring between 12 and 72 h, cells exhibited a decreased glycogen content together with a high glycolytic activity. Glycogen labelling, evaluated by 1 h-pulse experiments with [U-14C]glucose (5.5 mM), was minimal during the first 48 h and increased 2.5-fold at 72 h and 8-fold at 96 h, at which times it was also stimulated 2-fold by 10 nM insulin. [U-14C]Glucose carbons were incorporated into nucleic acid bases, with maximal incorporation at 72 h, the rate of nucleotide base labelling exceeding that of glycogen during the first 2 days of culture. Incubation of the cells with [U-14C]glucose resulted in the release into the medium of 14C-labelled glycine, the first intermediate formed on the route from serine to DNA. The rate of release per cell decreased as a function of cell growth, concomitantly with an increased rate of glucose carbon incorporation into nucleotide bases. The latter implied the intermediary formation of amino acids since the transaminase inhibitor cycloserine (10 mM), which totally inhibited [14C]glycine release, decreased by 65% nucleotide labelling from [U-14C]glucose. A dose-dependent inhibition by serine of the rate of [U-14C]glucose carbon incorporation into nucleotide bases was observed, which was maximal at 5 mM serine. These metabolic flux measurements indicate that glucose can be used as a precursor of nucleic acid synthesis. These results strongly suggest that this process is to a large extent mediated by a serine/glycine-biosynthesis-mediated pathway, and reinforce the hypothesis that glycolysis contributes to enhancing the provision of precursors required for cell proliferation.

Interchange of amino acids between tumor and host

During the growth of a tumor, there are very relevant changes in the metabolism of the host to produce the metabolites rapidly consumed by the tumor. In this context, the exchanges of amino acids between the tumor and its host are especially important; however, they have received little attention. A rigorous study must provide data on the growth curve of the tumor, as well as on amino acid levels in tumor cells, plasma, and metabolically relevant tissues and organs from the host during the whole growth of the tumor. The main conclusions arising from a complete study in a tumor model are discussed.

Sodium-dependent L-serine transport in plasma membrane vesicles isolated from Ehrlich cells by two-phase compartmentation

Plasma membrane vesicles were prepared from Ehrlich cells using two-phase system compartmentation. The highly pure plasma membrane vesicles obtained presented a negligible mitochondrial contamination and were suitable for studies of amino acid transport. L-Serine transport showed a clear ionic specificity, maximum incorporation being observed when an inwardly directed NaSCN gradient was used. Na(+)-dependent L-serine transport was dependent on assay temperature and membrane potential, and it seemed to be carried out by two different transport systems. An essential sulfhydryl group seemed to be involved in the transport process.

Role of serine biosynthesis and its utilization in the alternative pathway from glucose to glycogen during the response to insulin in cultured foetal-rat hepatocytes

The role of serine as a possible intermediate of the alternative pathway from glucose to glycogen was investigated under basal and insulin-stimulated conditions in 18-day cultured foetal-rat hepatocytes because these cells cannot use pyruvate-derived metabolites [Bismut & Plas (1989) Biochem. J. 263, 889-895]. Incubation of cells with [U-14C]glucose for 24 h led to a release of labelled serine in the medium concomitantly with a net serine production (100 nmol/24 h per culture). The rate of [14C]serine formation (close to 3 nmol/h per culture) indicated that a large part of newly formed serine originated from glucose. When short-term experiments were performed at day 2, glycogen labelling from [U-14C]serine or [U-14C]glycine, which was increased 3-fold by insulin after 2 h, evidenced their participation as glycogenic precursors. When a double-isotope procedure with [U-14C,3-3H]glucose was used, the direct and the alternative pathways from glucose were found to contribute to glycogenesis by 75 and 25% respectively. Cycloserine (18 mM), a transaminase inhibitor, strongly inhibited glycogen labelling from [U-14C] serine while producing a 70% increase in glucose incorporation by the alternative pathway, in both the presence and the absence of insulin. The inhibitor had no effect on the direct pathway from glucose to glycogen. Supplementation with 1 mM-hydroxypyruvate, a serine-derived metabolite, did not affect direct glucose incorporation, whereas the alternative pathway was stimulated whether insulin was present or not. These results indicate that the sequence glucose----serine----glycogen is operative in cultured foetal hepatocytes. The alternative pathway interferes with hydroxypyruvate utilization, and is likely mediated by the serine aminotransferase pathway, independently of the acute glycogenic action of insulin.

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.

Rapid and steady-state amino acid transport in perfused human fibroblasts and colon adenocarcinoma cells: effects of methotrexate

Initial and steady-state uptakes of serine and phenylalanine by human fibroblasts and human colon tumour cells were studied applying a double isotope dilution technique to perfused populations of cultivated cells retained on microcarrier beads. This new method permits the differentiation of the unidirectional transport parameters and can also distinguish between membrane-associated processes and independently intracellular events in isolated cells. High initial L-serine uptake values in colon adenocarcinoma cells became negative under steady-state conditions. To determine if the observed negative L-serine uptake was produced by the rapid efflux of intracellular L-[3H]serine, the cells were treated with methotrexate (MTX) (an inhibitor of cytosolic dihydrofolate reductase). The modified curve of L-[3H]serine uptake after MTX treatment suggests that, under these experimental conditions, net serine transport is non concentrative in colon tumour cells and could be modulated by the rate of intracellular serine metabolism; it also suggests that MTX does not directly affect serine transport in perfused human colon adenocarcinoma cells. Initial and steady-state uptakes of phenylalanine were high in both fibroblasts and tumour cells and were unaffected by MTX treatment.

Metabolic control analysis of mammalian serine metabolism

(1) Mammalian serine metabolism is discussed in relation to its synthesis and utilization in proliferating cells, particularly during the nonmalignant proliferation of lymphocytes. (2) An analysis of the control of serine biosynthesis de novo under conditions of high pathway flux has been carried out using metabolic control theory. (3) The important and novel conclusions are that control of pathway flux is localized exclusively at the final step of this biosynthetic pathway, phosphoserine phosphatase. This conclusion challenges the frequently stated maxim that control of biosynthetic pathways is always directed at the first pathway enzyme in a sequence. In the case of phosphoserine phosphatase, the enzyme is inhibited uncompetitively by its product serine, and this feedback control mechanism has the most significant controlling influence on overall pathway flux. Thus, the serine biosynthesis pathway, under these conditions, is controlled by product demand (serine utilization) and not by substrate supply (glycolytic provision of 3-phosphoglycerate), despite the high rate of glycolysis associated with cell proliferation. (4) The control structure of the pathway is not immutable. As has been observed with other pathways analyzed by metabolic control theory, the key points of control in the pathway can shift according to physiological circumstances. At low pathway flux, the control of serine biosynthesis is shared between all the component enzymes of the pathway, and the responsiveness of flux shifts from product demand to substrate supply. (5) Serine utilization has been studied in mitogenically-stimulated human peripheral lymphocytes. Cell proliferation and serine utilization for nucleic acid synthesis have been shown to be responsive to serine concentrations in the normal plasma range. (6) It is concluded that the maintenance of normal plasma serine concentrations is an important factor in the rate of lymphocyte proliferation and hence the effectiveness with which the body can mount an immune response to an antigenic challenge, such as in infection.

Enzymic imbalance in serine metabolism in human colon carcinoma and rat sarcoma

The activities of 3-phosphoglycerate dehydrogenase, an enzyme of serine biosynthesis, and serine hydroxymethyltransferase, serine dehydratase and serine aminotransferase, which are competing enzymes of serine utilization, were assayed in human colon carcinomas from patients and in transplantable rat sarcomas. Serine dehydratase and serine aminotransferase activities were absent, whereas 3-phosphoglycerate dehydrogenase and serine hydroxymethyltransferase activities were markedly increased in both tumour types. Serine hydroxymethyltransferase catalyses the formation of glycine and methylene tetrahydrofolate which are important precursors for nucleotide biosynthesis. The observed enzymic imbalance in these tumours ensures that an increased capacity for the synthesis of serine is coupled to its utilisation for nucleotide biosynthesis as a part of the biochemical commitment to cellular replication in cancer cells. That this pattern is found in sarcomas and carcinomas, and in tumours of human and rodent origin, signifies its universal importance for the biochemistry of the cancer cell and singles it out as a potential target site for anti-cancer chemotherapy.

The modulation of serine metabolism in hepatoma 3924A during different phases of cellular proliferation in culture

The activities of 3-phosphoglycerate dehydrogenase and serine hydroxymethyltransferase increased markedly during the transition of hepatoma cells from a resting non-proliferating culture into the proliferating growth phase. Activities declined as cells reached confluency and entered the plateau growth phase. This pattern was paralleled by changes in [14C]serine incorporation into nucleic acids. The experiments support the hypothesis that the biosynthesis of serine is metabolically coupled to its utilization for nucleotide precursor formation in cancer cells.

Cerebral glycine content and phosphoserine phosphatase activity in hyperaminoacidemias

Chronic hyperphenylalaninemia maintained with the aid of a suppressor of phenylalanine hydroxylase, alpha-methylphenylalanine, increases the glycine concentration and the phosphoserine phosphatase activity of the developing rat brain but not that of liver or kidney. Similar increases occur after daily injections with large doses of phenylalanine alone, while tyrosine, isoleucine, alanine, proline, and threonine, were without effect. Treatment with methionine, which increases the phosphoserine phosphatase activity of the brain and lowered that of liver and kidney, left the cerebral glycine level unchanged. When varying the degrees of gestational or early postnatal hyperphenylalaninemia, a significant linear correlation was found between the developing brains' phosphoserine phosphatase and glycine concentration. Observations on the uptake of injected glycine and its decline further indicate that coordinated rises in the brain's phosphoserine phosphatase and glycine content associated with experimental hyperphenylalaninemia denote a direct impact of phenylalanine on the intracellular pathway of glycine synthesis in immature animals.

Enzymic imbalance in serine metabolism in rat hepatomas

The activity of 3-phosphoglycerate dehydrogenase was high in tissues of high cell-renewal capacity, and was increased in neonatal and regenerating liver and, more markedly, in hepatomas. Serine hydroxymethyltransferase activity was present in hepatomas, whereas other enzymes of serine utilization (serine dehydratase and serine aminotransferase) were absent. This enzymic imbalance couples serine biosynthesis preferentially to nucleotide precursor formation in cancer cells.

Enzymes of serine metabolism in normal and neoplastic rat tissues

Enzymes involved in the pathway of de novo serine biosynthesis (L-phosphoserine aminotransferase) and in alternative pathways of serine utilization (L-serine hydroxymethyltransferase, L-serine dehydratase and L-serine aminotransferase) were assayed in normal adult and fetal rat tissues and in a range of transplantable rat tumors. Serine dehydratase and serine aminotransferase activities were essentially confined to normal adult liver and kidney, whereas phosphoserine aminotransferase and serine hydroxymethyltransferase activities showed a more ubiquitous tissue distribution. In particular, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were appreciable in neoplastic tissues, in the absence of the other enzymes of serine utilization. The pattern of enzyme distribution suggests that the synthesis of serine de novo is metabolically coupled to its utilization for nucleotide biosynthesis in tumors of differing tissue origins.

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)

Enzyme pathology and the histologic categorization of human lung tumors: the continuum of quantitative biochemical indices of neoplasticity

The purpose of the present enzymic and histologic analysis of pulmonary samples from 39 subjects was to discern a common, meaningful pattern which may underlie the biochemical heterogeneity of lung neoplasms. The distribution among the different tumors of thymidine kinase, uridine kinase, phosphoserine phosphatase, hexokinase and adenylate kinase was found to correlate with each other. By averaging their standardized units (normal lung = 0) an enzymic index of neoplasticity was calculated for each tumor and used (in increasing order) to rank all 39. The index, showing a significant positive correlation with mitotic frequency, encompassed a continuous 100-fold range. Poorly differentiated carcinomas ranked high while neoplasms with better differentiation and prognosis placed in the lower half of the range. The results indicate that enzymes showing coordinated variations over a broad spectrum of tumors could contribute objective criteria to the rating of any individual tumor against a continuous, quantitative scale of neoplasticity.

Chronic hyperphenylalaninemia produces cerebral hyperglycinemia in immature rats

The amino acid content of three tissues was measured in 10-day-old rats made hyperphenylalaninemic from age 3 to 10 days by daily injection of phenylalanine plus alpha-methylphenylalanine to inhibit phenylalanine hydroxylase (PAH). At 12 h after the last injection, the concentrations of alanine, valine, methionine, isoleucine, and leucine in the cerebral hemispheres were depressed by 25-50%, whereas that of glycine was elevated 2.3-fold. In the spinal cord, the levels of phosphoserine, methionine, and leucine were decreased by 40-50%, and those of serine and threonine increased by 50%. Tyrosine and phenylalanine concentrations were high in all tissues, 2-3 and 15-30 times normal, respectively; of the amino acids investigated, they were the only ones changed in the liver. Cerebral hyperglycinemia was also produced by chronic treatment with phenylalanine plus p-chlorophenylalanine to inhibit PAH, but not by acute (12 h) hyperphenylalaninemia. An increase in cerebral phosphoserine phosphatase activity was greater in rats treated with phenylalanine plus PAH inhibitor than with inhibitor alone. The content of brain glycine normally declines with age from birth to 15 days; this decrease was prevented by chronic hyperphenylalaninemia. Attempts to reduce the cerebral glycine content of the hyperphenylalaninemic rats were unsuccessful. However, one of the therapeutic protocols, methionine loading, may be useful because it increased the methionine and decreased the phenylalanine contents in the brain.

The effect of hyperphenylalaninaemia on glycine metabolism in developing rat brain

The brains of 3--16-day-old rats that were rendered hyperphenylalaninaemic by daily injections of alpha-methylphenylalanine plus phenylalanine were subjected to biochemical analysis. Fluctuations throughout the treatment period in the concentrations of branched-chain amino acids, methionine and serotonin were in agreement with the known interference of excess plasma phenylalanine with transport. The glycine content, however, became abnormal only by day 5, remained so through the treatment, and the elevation was equally apparent at 4, 8 or 24 h after the last daily injections. On the last day of treatment there were small increases in the taurine, glutamate, aspartate and 4-aminobutyrate concentrations, attributable mainly to the diencephalon or brain stem. After day 3 of treatment there were persistent elevations in the specific activity of phosphoserine phosphatase and glycine synthase (but not serine hydroxymethyltransferase) of the brain in each of the regions analysed. The observations indicate that chronic hyperphenylalaninaemia interferes with the normal regulation of intracerebral glycine metabolism during a critical period of early postnatal development, and suggest that the resulting excess in this amino acid (particularly marked in the cortex) contributes to the behavioural abnormalities that these animals exhibit in later life.

Enzyme activities in human fetal and neoplastic tissues

The concentrations of ten or 12 enzymes involved in the metabolism of DNA, collagen, amino acids, or glucose have been determined in variants of human intestinal and pulmonary tissues. In comparison to nonneoplastic adult colon, normal fetal colon had elevated concentrations of thymidine kinase, peptidyl proline hydroxylase, phosphoserine phosphatase, ornithine transcarbamylase, gamma-glutamyl transpeptidase, and ornithine aminotransferase. Raised activities of the first five of these enzymes, and of hexokinase, glucose-6-phosphate dehydrogenase, and pyrroline-5-carboxylate reductase distinguishes neoplastic from nonneoplastic sections of adult colon. Study of a wide range of pulmonary specimens permitted comparisons of different types of tumors, and revealed some subtle differences between lungs of noncancer patients and nonneoplastic portions of host lungs. The concentrations of eight previously identified enzymic indicators were less in moderately or well differentiated than in poorly differentiated pulmonary adenocarcinomas. The latter differed from epidermoid carcinomas (also poorly differentiated) by containing lower concentrations of thymidine kinase (both soluble and particulate) and hexokinase.

Liver enzymes of serine metabolism during neonatal development of the rat

The developmental patterns of L-serine hydroxymethyltransferase, L-phosphoserine aminotransferase, L-serine aminotransferase and L-serine dehydratase were determined in rat liver. The results point to an increased capacity for serine biosynthesis de novo in the perinatal period. It is suggested that serine at this time, and also at weaning, may serve as a precursor, via the serine hydroxymethyltransferase reaction, for nucleotide biosynthesis to support the rapid phases of liver growth. The role of the alternative pathways of serine metabolism during neonatal development is discussed.

Human colon tumors: enzymic and histological characteristics

In samples of colonic adenocarcinomas, the mean activities of thymidine kinase, glucose-6-phosphate dehydrogenase, phosphoserine phosphatase and pyrroline-5-carboxylate reductase were several fold higher than those of nonneoplastic colon. The presence of considerable, cold labile pyrroline-5-carboxylate reductase activity provided an additional criterion for distinguishing tumors from the control tissue. Deviations from the pattern of enzymes in normal colon were much more pronounced in the five moderately well-differentiated than in the single well-differentiated adenocarcinoma.