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K Snell

Publications and source records attributed to K Snell.

At least 37 records · Page 2Linked to original sources
PubMed1992

Nucleotide sequence and expression of a cDNA encoding rabbit liver cytosolic serine hydroxymethyltransferase.

A rabbit liver cDNA library in phage lambda gt10 was screened using a portion of the coding sequences for rabbit cytosolic serine hydroxymethyltransferase (amino acids 244-420) that had been amplified by PCR, with total rabbit liver RNA as a template. A clone of 2.3 kb (pUS1203) was isolated and the nucleotide sequence showed that it contained an open reading frame of 1452 bp, which coded for serine hydroxymethyltransferase and was flanked by 155 bp at the 5' end and 653 bp at the 3' end. The full-length cDNA was cloned into an expression vector and transfected into COS-1 cells. Serine hydroxymethyltransferase activity was increased by 33% in the transfected cells and a new protein band of the appropriate size was seen by SDS/PAGE analysis of proteins extracted from transfected cells. The protein sequence for rabbit cytosolic serine hydroxymethyltransferase derived from the cDNA nucleotide sequence was compared with three other derived or known prokaryotic and eukaryotic sequences. An overall sequence similarity of 34% was noted between all four sequences, whereas the similarity between the rabbit cytosolic and mitochondrial isoforms was 62%.

Amino Acid Sequence↗
PubMed1992

Embryo implantation rates in oocyte donation: a prospective comparison of tubal versus uterine transfers.

OBJECTIVE: To compare pregnancy and implantation rates in tubal and uterine transfers during a hormonal replacement cycle in an oocyte donation program. DESIGN: Prospective randomized. PATIENTS: Forty-two consecutive patients who entered an oocyte donation program. INTERVENTIONS: Twenty-two patients were assigned for uterine transfer and 20 for tubal embryo transfer (ET). RESULTS: Twenty-three pregnancies were achieved, 12 (54.5%) after uterine transfers and 11 (57.9%) after tubal transfers. Implantation rates in both groups are not significantly different (17.4% uterine transfers versus 21.5% tubal ETs). CONCLUSIONS: Our results suggest that in hormonal replacement cycles (uniform endometrial stimulation) there is no advantage in transferring embryos to the fallopian tube. Furthermore, embryo quality and endometrial receptivity appear to be significantly more important than the time of entrance of an embryo to the uterine cavity in determining its chances of implantation.

Embryo Implantation↗
PubMed1990

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.

Animals↗
PubMed1989

Effects of a triazine antifolate (NSC 127755) on serine hydroxymethyltransferase in myeloma cells in culture.

The effect of the triazine antifolate NSC 127755 on serine hydroxymethyltransferase activity in mouse myeloma X63 cells in culture was determined. The enzyme was inhibited, apparently irreversibly, with IC50 approximately 5 X 10(-8) M. A similar concentration-dependency for inhibition of [6-3H]deoxyuridine incorporation into DNA was observed in these cells in culture. Long-term culture of myeloma cells with NSC 127755 resulted in a progressive decrease in the number of viable cells. The study emphasises the significance of serine hydroxymethyltransferase as a target for anticancer chemotherapy.

Animals↗
PubMed1988

Control analysis of mammalian serine biosynthesis. Feedback inhibition on the final step.

The flux of serine biosynthesis in the liver of the normal rabbit, and of the rat on a low protein diet, is most sensitive to the activity of phosphoserine phosphatase (flux control coefficient up to 0.97), the last of the three enzymes in the pathway after it branches from glycolysis. The concentration of the pathway product, serine, has a strong controlling influence on the flux (response coefficient up to -0.64) through feedback inhibition at this step. The pathway is therefore controlled primarily by the demand for serine rather than the supply of the pathway precursor, 3-phosphoglycerate. Under conditions where there is a lower biosynthetic flux, the flux control coefficients of the first two enzymes of the pathway are increased, and are probably dominant in the rat on a normal diet. In rabbit liver, when ethanol is used to inhibit serine biosynthesis, control can be distributed between the three enzymes, even though the reactions catalysed by the first two remain close to equilibrium. Apart from their intrinsic value in aiding the understanding of the regulation of mammalian serine metabolism, our findings illustrate the danger of assuming that there are invariant design principles in the regulation of metabolic pathways, such as feedback control on the first step after a branch.

Animals↗
PubMed1988

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.

Aged↗
PubMed1988

Characterization of rat liver beta-adrenoceptors during perinatal development as determined by [125I]-iodopindolol radioligand binding assays.

1. The subtype specificity of beta-adrenoceptors in foetal (20 days post coitum) rat liver membrane preparations has been determined by use of [125I]-iodopindolol binding assays and the characteristics of radioligand binding have been resolved. 2. The kinetics of radioligand association and dissociation (in the presence of 5 x 10(-4) M isoprenaline) showed an association rate constant of 1.5 x 10(7) M-1 S-1 and dissociation rate constant of 9.1 x 10(-4) S-1, corresponding to a dissociation constant for [125I]-iodopindolol of 60.7 pM. A similar dissociation constant (75 pM) was determined by saturation binding assays. 3. The rank order of potency for displacement of [125I]-iodopindolol binding was consistent with binding to a predominantly beta 2-adrenoceptor population (i.e. ICI 118551 greater than isoprenaline greater than adrenaline greater than noradrenaline greater than atenolol). Computer analysis of displacement curves in the presence of a beta 1-subtype selective agent (atenolol) or a beta 2-subtype selective agent (ICI 118551) revealed the presence of beta 2- and beta 1-adrenoceptor subtypes in a ratio of about 80:20%. 4. Saturation binding assays by use of [125I]-iodopindolol were carried out at different perinatal ages to determine total beta-adrenoceptor concentrations and beta 2-subtype (in the presence of 5 x 10(-7) M atenolol) adrenoceptor concentrations. Competition binding assays with atenolol confirmed that at all ages apparent beta 2-adrenoceptor binding accounted for 84-95% of the total beta-adrenoceptor binding. The total beta- and beta 2-adrenoceptor binding capacity increased by 2.3 fold from 20 days post coitum to birth, and then decreased postnatally at 1 and 2 days post partum. The dissociation constant for [125I]-iodopindolol binding did not show any change with age. 5. The change in beta 2-adrenoceptor concentration with age is discussed in relation to the changing beta-adrenoceptor-mediated responsiveness of glucose production by rat liver during perinatal development.

Animals↗
PubMed1987

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.

Animals↗
PubMed1986

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.

Animals↗
PubMed1985

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.

Animals↗
PubMed1985

Branched-chain amino acid metabolism and alanine formation in rat muscles in vitro. Mitochondrial-cytosolic interrelationships.

Muscle branched-chain amino acid metabolism is coupled to alanine formation via branched-chain amino acid aminotransferase and alanine aminotransferase, but the subcellular distributions of these and other associated enzymes are uncertain. Recovery of branched-chain aminotransferase in the cytosol fraction after differential centrifugation was shown to be accompanied by leakage of mitochondrial-matrix marker enzymes. By using a differential fractional extraction procedure, most of the branched-chain aminotransferase activity in rat muscle was located in the mitochondrial compartment, whereas alanine aminotransferase was predominantly in the cytosolic compartment. Phosphoenolpyruvate carboxykinase, like aspartate aminotransferase, was approximately equally distributed between these subcellular compartments. This arrangement necessitates a transfer of branched-chain amino nitrogen and carbon from the mitochondria to the cytosol for alanine synthesis de novo to occur. In incubations of hemidiaphragms from 48 h-starved rats with 3mM-valine or 3mM-glutamate, the stimulation of alanine release was inhibited by 69% by 1 mM-aminomethoxybut-3-enoate, a selective inhibitor of aspartate aminotransferase. Leucine-stimulated alanine release was unaffected. These data implicate aspartate aminotransferase in the transfer of amino acid carbon and nitrogen from the mitochondria to the cytosol, and suggest that oxaloacetate, via phosphoenolpyruvate carboxykinase, can serve as an intermediate on the route of pyruvate formation for muscle alanine synthesis.

Alanine↗
PubMed1985

Alanine and inter-organ relationships in branched-chain amino and 2-oxo acid metabolism. Review.

Branched-chain amino acid metabolism in skeletal muscle promotes the production of alanine, an important precursor in hepatic gluconeogenesis. There is controversy concerning the origin of the carbon skeleton of alanine produced in muscle, specifically whether it is derived from carbohydrate via glycolysis (the glucose-alanine cycle) or from amino acid precursors (viz. glutamate, valine, isoleucine, methionine, aspartate, asparagine) via a pathway involving phosphoenolpyruvate (PEP) carboxykinase and pyruvate kinase, or NADP-malate dehydrogenase (malic enzyme). The relevant literature is reviewed and it is concluded that neogenic flux from amino acids is unlikely to be of major quantitative importance for provision of the carbon skeleton of alanine either in vitro or in vivo. Evidence is presented that branched-chain amino acid oxidation in muscle is incomplete and that the branched-chain 2-oxo acids and the products of their partial oxidation (including glutamine) are released. The role of these metabolites is discussed in the context of fuel homeostasis in starvation.

Alanine↗