Synthesis and degradation of tyrosinase in cultured melanoma cells

The tyrosinase (EC 1.14.18.1) activity of cultured B-16 mouse melanoma cells (C2M) in the stationary phase depends greatly on whether the culture medium contains glucose or galactose. The activity in medium containing galactose was about ten times that in medium containing glucose at pH 7.2. This difference in tyrosinase activity was concluded to be due to a shift of balance between synthesis and degradation of the enzyme. Experiments were conducted with stationary phase cultures in the presence of cycloheximide. The melanoma cells did not synthesize tyrosinase in medium containing glucose in the stationary phase. But when they were cultured under identical conditions, except that glucose was replaced by galactose, they continued to synthesize tyrosinase. The rate of synthesis in medium containing galactose at pH 6.3 was one third of that in the same medium at about pH 7, in which the increase in specific activity of tyrosinase per day was about 30 nmoles/mg cell protein per hr. The rate of degradation of the enzyme was practically the same in medium containing glucose as in medium containing galactose, and largely depended on the pH of the culture medium. At pH 6.3, the half-life was about one third of that at pH 7.2, where it was about 1.8 days. The degradation at acidic pH values was much reduced by ammonium salt and was strongly inhibited by the protease inhibitor, leupeptin.     

The effect of pH on the propagation of the diploid cell WI-38 in galactose medium

The pH range for optimal growth of WI-38 in Eagle's minimum essential medium varies with the carbohydrate utilized. Growth with glucose occurred only in the pH range 6.8–7.4. Glucose-free galactose supported growth only if the pH of the medium was adjusted to 6.4–6.8. Supplementing the galactose medium with pyruvate extended the pH range for growth to 7.4. The addition of 0.01% glucose to the galactose medium increased growth at pH 7.0–7.4, reduced growth at pH 6.8, and completely inhibited growth at pH 6.6. This toxic effect of glucose under conditions impairing glycolysis was considered to be a contributing factor to the post-confluent growth rate decline associated with “contact inhibition.”     

Comparative metabolic analysis of lactate for CHO cells in glucose and galactose

t-PA producing CHO cells have been shown to undergo a metabolic shift when the culture medium is supplemented with a mixture of glucose and galactose. This metabolic change is characterized by the reincorporation of lactate and its use as an additional carbon source. The aim of this work is to understand lactate metabolism. To do so, Chinese hamster ovary cells were grown in batch cultures in four different conditions consisting in different combinations of glucose and galactose. In experiments supplemented with glucose, only lactate production was observed. Cultures with glucose and galactose consumed glucose first and produced lactate at the same time, after glucose depletion galactose consumption began and lactate uptake was observed. Comparison of the metabolic state of cells with and without the shift by metabolic flux analysis show that the metabolic fluxes distribution changes mostly in the reactions involving pyruvate metabolism. When not enough pyruvate is being produced for cells to support their energy requirements, lactate dehydrogenase complex changes the direction of the reaction yielding pyruvate to feed the TCA cycle. The slow change from high fluxes during glucose consumption to low fluxes in galactose consumption generates intracellular conditions that allow the influx of lactate. Lactate consumption is possible in cell cultures supplemented with glucose and galactose due to the low rates at which galactose is consumed. Evidence suggests that an excessive production and accumulation of pyruvate during glucose consumption leads to lactate production and accumulation inside the cell. Other internal conditions such as a decrease in internal pH, forces the flow of lactate outside the cell. After metabolic shift the intracellular pool of pyruvate, lactate and H⁺ drops permitting the reversal of the monocarboxylate transporter direction, therefore leading to lactate uptake. Metabolic analysis comparing glucose and galactose consumption indicates that after metabolic shift not enough pyruvate is produced to supply energy metabolism and lactate is used for pyruvate synthesis. In addition, MFA indicates that most carbon consumed during low carbon flux is directed towards maintaining energy metabolism.     

Glutamine dependency of human skin fibroblasts: modulation by hexoses

The combined effects of carbohydrates and glutamine were investigated in diploid strains of normal human skin fibroblasts cultured for 21 days under eight different culture conditions: hexose-free medium or medium containing D-glucose, D-galactose, or D-fructose, with or without added glutamine. Cell growth, hexose consumption, lactate production, intracellular glycogen content and extracellular amino acid levels were measured every third to fourth day. In the presence of glutamine, cells reached a higher saturation density in fructose medium than in glucose or galactose medium but per cell consumption of fructose and galactose was much less than that of glucose. Consumption of all three carbohydrates per unit cell growth exhibited three distinct phases: Days 1-3, 3-10, and 10-20, respectively. In the absence of glutamine the rate of cell growth was not altered in glucose or galactose medium, but slowed down considerably in fructose medium. Glutamine deprivation also led to changes in hexose consumption. In hexose-free media the cell growth rate at first was very slow, but rose after 2 or 3 weeks of culture. The levels of extracellular nonessential amino acids varied according to medium and growth phase. One of the most exciting findings was that human fibroblasts are able to maintain a slight excess of glutamine in all media not supplemented with glutamine and, more surprisingly, to synthesize it in a medium containing galactose and glutamine.     

Growth Profile of Crown Gall Cells of Tobacco in Suspension Culture

In order to obtain a basic information of plant cell suspension culture as a step toward the development of large scale culture, culture conditions of crown gall cells (auxin non-requiring cells) were investigated. Addition of yeast extract to culture medium was significantly effective for the growth and cell dispersion.

In experiments on the ability of the cultured cells to utilize sugars as the carbon source, it was observed that galactose, added to the culture medium, markedly inhibited the cell growth.

Pasteurization of the medium containing fructose as carbon source made it brownish by Maillard reaction and the medium apparently restrained the cell growth. However, the fructose medium sterilized by filtration was excellent for the cell growth as well as sucrose or glucose medium. In a jar fermentor, even the glucose medium became brownish by heat sterilization and the brown colored medium restrained the cell growth. Under optimum conditions, the doubling time was 1.1 day in exponential phase and 2.0 g of cell (dry weight) per 100 ml culture was obtained as the maximum yield.     

Microbial immunomodulators: mannoprotein constituents of<Emphasis Type="Underline">Candida albicans</Emphasis> with immunomodulatory activity in lymphocyte cultures of normal,glioma-bearing and HIV-infected subjects

The culture kinetics of human tumor kidney cells (TCL 598) grown on microcarriers are compared with media initially supplemented with either glucose alone or a mixture of galactose and glucose. Growth rates and maximal cell densities are similar, but cellular death is much slower in galactose than in glucose. Galactose is metabolized at a much slower specific rate than glucose. Cells grown in the galactose medium show a different pattern of lactate and pyruvate metabolism compared to cells grown in the glucose medium. Growth with galactose also favours oxidation of glutamine to alanine.     

Comparative effects of glucose and fructose on growth and morphological aspects of cultured skin fibroblasts

The growth rate of human skin fibroblasts was evaluated when glucose was replaced by fructose in the culture medium. Four mediums containing respectively 5.5 mmol/l glucose (G1), 27.5 mmol/l glucose (G5), 5.5 mmol/l fructose (F1), and 27.5 mmol/fructose (F5) were used. Skin fibroblasts from fourteen subjects were continuously cultured for 20 days and the number of cells was counted at days 1, 3, 7, 10, 15 and 20 after plating. The morphological patterns were observed and compared, the pH values of the medium were calculated, as were hexose consumption and lactate production. The results established clear differences in cell growth, pH and morphology: up to day 7, the growth rate was lower in fructose than in glucose medium, and the pH values were higher. In addition, marked steatosis appeared, with increased pyruvate dehydrogenase (PDH) activity. After day 10, the mean values gave a significant increase in the number of cells grown in fructose mediums, even if variations occurred between different cell strains. This increase was accompanied by loss of density-dependent growth inhibition and a reduction in the quantity and size of the vacuoles caused by steatosis. These findings were also established for other cell types, like aponeurosis fibroblasts. In addition, the longevity of the strains increased. These observations indicate that intermediary metabolism is considerably influenced by the carbohydrate present in the cell culture medium and that there are also repercussions on the growth rate. Under our experimental conditions, metabolism pathways seemed to differ on day 7 and on day 20. The various metabolic events suggested by the differences in the pH values are now being studied in our laboratory.     

Initiation of Yeast Sporulation by Partial Carbon, Nitrogen, or Phosphate Deprivation

In this paper we show that partial deprivation of a carbon source, a nitrogen source, or phosphate in the presence of all other nutrients needed for growth initiates meiosis and sporulation of Saccharomyces cerevisiae homothallic strain Y55. For carbon deprivation experiments, cells were grown in synthetic medium (pH 5.5) containing an excess of one carbon source and then transferred to the same medium containing different concentrations of the same carbon source. In the case of transfer to different acetate concentrations, the log optical density at 600 nm increased at the previous rate until the cells had used up all of the acetate, whereupon the cells entered a stationary phase and did not sporulate. The same was observed with ethanol. In contrast, at different concentrations of dihydroxy-acetone or pyruvate, cells grew at different rates and sporulated optimally at intermediate concentrations (50 to 75 mM). The response to galactose was similar but reflected the presence of a low-affinity galactose transport system and the induction of a high-affinity galactose transport system. Cells could also sporulate when a glucose medium ran out of glucose, apparently because they initiated sporulation during the subsequent lag period and then used the produced ethanol as a carbon source. For phosphate deprivation experiments, cells growing with excess ethanol or pyruvate and phosphate were transferred to the same medium containing limiting amounts of phosphate. First, they used up the intracellular phosphate reserves for rapid growth, and then they sporulated optimally when an intermediate concentration (30 μM) of phosphate had been added to the medium. For nitrogen deprivation experiments, cells grown with excess acetate, ethanol, or pyruvate and NH₄⁺ were transferred to the same medium from which all nitrogen had been removed. These cells sporulated well in acetate medium but poorly in ethanol and pyruvate media. However, the sporulation frequency in the latter media could be increased greatly by adding intermediate concentrations (1 mM) of the slowly metabolizable amino acids glycine, histidine, or phenylalanine. If one assumes that the sporulation response to partial deprivation of carbon-, nitrogen-, or phosphorus-containing compounds reflects control by a single metabolite, the intracellular concentration of this metabolite may decide at the START position (G1 phase) of the cell cycle whether a/α cells enter mitosis or meiosis.     

Catabolite inhibition and sequential metabolism of sugars by Streptococcus lactis.

Growth of galactose-adapted cells of Streptococcus lactis ML(3) in a medium containing a mixture of glucose, galactose, and lactose was characterized initially by the simultaneous metabolism of glucose and lactose. Galactose was not significantly utilized until the latter sugars had been exhausted from the medium. The addition of glucose or lactose to a culture of S. lactis ML(3) growing exponentially on galactose caused immediate inhibition of galactose utilization and an increase in growth rate, concomitant with the preferential metabolism of the added sugar. Under nongrowing conditions, cells of S. lactis ML(3) grown previously on galactose metabolized the three separate sugars equally rapidly. However, cells suspended in buffer containing a mixture of glucose plus galactose or lactose plus galactose again consumed glucose or lactose preferentially. The rate of galactose metabolism was reduced by approximately 95% in the presence of the inhibitory sugar, but the maximum rate of metabolism was resumed upon exhaustion of glucose or lactose from the system. When presented with a mixture of glucose and lactose, the resting cells metabolized both sugars simultaneously. Lactose, glucose, and a non-metabolizable glucose analog (2-deoxy-d-glucose) prevented the phosphoenolpyruvate-dependent uptake of thiomethyl-beta-d-galactopyranoside (TMG), but the accumulation of TMG, like galactose metabolism, commenced immediately upon exhaustion of the metabolizable sugars from the medium. Growth of galactose-adapted cells of the lactose-defective variant S. lactis 7962 in the triple-sugar medium was characterized by the sequential metabolism of glucose, galactose, and lactose. Growth of S. lactis ML(3) and 7962 in the triple-sugar medium occurred without apparent diauxie, and for each strain the patterns of sequential sugar metabolism under growing and nongrowing conditions were identical. Fine control of the activities of preexisting enzyme systems by catabolite inhibition may afford a satisfactory explanation for the observed sequential utilization of sugars by these two organisms.     

Isolation of dicarboxylic acid- and glucose-binding proteins from Pseudomonas aeruginosa.

Inducible binding proteins for C4-dicarboxylic acids (DBP) and glucose (GBP) were isolated from Pseudomonas aeruginosa by extraction of exponential-phase cells with 0.2 M MgC12 (pH 8.5) and by an osmotic shock procedure without affecting cell viability. DBP synthesis was induced by growth on aspartate, alpha-ketoglutarate, succinate, fumarate, malate, and malonate but not by growth on acetate, citrate, pyruvate, or glucose. Binding of succinate by DBP was competitively inhibited by 10-fold concentrations of fumarate and malate but not by a variety of related substances. GBP synthesis and transport of methyl alpha-glucoside by whole cells were induced by growth on glucose or pyruvate plus galactose, 2-deoxyglucose, or methyl alpha-glucoside but not by growth on gluconate, succinate, acetate, or pyruvate. The binding of radioactive glucose by GBP was significantly inhibited by 10-fold concentrations of glucose, galactose, and glucose-1-phosphate but not by the other carbohydrates tested. The binding of glucose by GBP or succinate by DBP did not result in any chemical alteration of the substrates.     

Influence of sugar source (lactose,glucose, galactose) on 2,3-butanediol production by Klebsiella oxytoca NRRL-B199

The ability of Klebsiella oxytoca NRRL-B199 to use either lactose or the mixture of glucose and galactose as substrate for the production of 2,3-butanediol was studied in batch fermentations with different conditions of aeration and pH. 2,3-butanediol was undetected, or present in minute concentration in the fermentation broths with lactose, while it was the main product from glucose+galactose with final concentrations of up to 18.8 g/l in media at pH 6.0. Under conditions optimal for 2,3-butanediol synthesis, when aeration limited growth, the rate of biomass growth was more tightly related to the aeration rate in lactose medium than in glucose+galactose medium. These relations suggest that the growth rate is very low on lactose but still considerable on glucose+galactose when aeration rate tends toward zero. Correspondingly, the metabolism is more oxidative in the former medium, yielding mainly acetate as product.Abbreviations CDW cell dry weight     

Role of Galactose or Glucose-1-Phosphate in Preventing the Lysis of Streptococcus diacetilactis

Cells of Streptococcus diacetilactis DRCI grown at 32 C in media containing glucose as the energy source were osmotically fragile and began to lyse immediately after growth was stopped (by the action of chloramphenicol or the exhaustion of glucose), unless they were then stabilized by hypertonic medium or spermine or by storage at low pH or low temperature, or both. In media containing excess glucose, with growth limited by exhaustion of some nutrient other than the energy source, the appearance of lysis was masked by the occurrence of a balance between lysis and synthesis. The osmotic fragility apparently resulted from inability of the organism to use glucose as an adequate precursor of galactosamine, and conditions of temperature and pH that promoted rapid growth on glucose were particularly conducive to the formation of cells that lysed readily. Growing the organism in media containing galactose, lactose, maltose, or glucose (at 17 C) as energy source resulted in the formation of cells that were resistant to lysis and richer in galactosamine than unstable cells formed on glucose at 32 C. The results indicate that the organism phosphorolyzes maltose to glucose plus beta-glucose-1-phosphate, and suggest that it can use the beta-glucose-1-phosphate in place of alpha-glucose-1-phosphate in the formation of cell materials.     

Mechanisms by Which Low Glucose Enhances the Cytotoxicity of Metformin to Cancer Cells Both In Vitro and In Vivo

Different cancer cells exhibit altered sensitivity to metformin treatment. Recent studies suggest these findings may be due in part to the common cell culture practice of utilizing high glucose, and when glucose is lowered, metformin becomes increasingly cytotoxic to cancer cells. In low glucose conditions ranging from 0 to 5 mM, metformin was cytotoxic to breast cancer cell lines MCF7, MDAMB231 and SKBR3, and ovarian cancer cell lines OVCAR3, and PA-1. MDAMB231 and SKBR3 were previously shown to be resistant to metformin in normal high glucose medium. When glucose was increased to 10 mM or above, all of these cell lines become less responsive to metformin treatment. Metformin treatment significantly reduced ATP levels in cells incubated in media with low glucose (2.5 mM), high fructose (25 mM) or galactose (25 mM). Reductions in ATP levels were not observed with high glucose (25 mM). This was compensated by enhanced glycolysis through activation of AMPK when oxidative phosphorylation was inhibited by metformin. However, enhanced glycolysis was either diminished or abolished by replacing 25 mM glucose with 2.5 mM glucose, 25 mM fructose or 25 mM galactose. These findings suggest that lowering glucose potentiates metformin induced cell death by reducing metformin stimulated glycolysis. Additionally, under low glucose conditions metformin significantly decreased phosphorylation of AKT and various targets of mTOR, while phospho-AMPK was not significantly altered. Thus inhibition of mTOR signaling appears to be independent of AMPK activation. Further in vivo studies using the 4T1 breast cancer mouse model confirmed that metformin inhibition of tumor growth was enhanced when serum glucose levels were reduced via low carbohydrate ketogenic diets. The data support a model in which metformin treatment of cancer cells in low glucose medium leads to cell death by decreasing ATP production and inhibition of survival signaling pathways. The enhanced cytotoxicity of metformin against cancer cells was observed both in vitro and in vivo.     

Low Glucose but Not Galactose Enhances Oxidative Mitochondrial Metabolism in C2C12 Myoblasts and Myotubes

Background

Substituting galactose for glucose in cell culture media has been suggested to enhance mitochondrial metabolism in a variety of cell lines. We studied the effects of carbohydrate availability on growth, differentiation and metabolism of C2C12 myoblasts and myotubes.

Methodology/Principal Findings

We measured growth rates, ability to differentiate, citrate synthase and respiratory chain activities and several parameters of mitochondrial respiration in C2C12 cells grown in media with varying carbohydrate availability (5 g/l glucose, 1 g/l glucose, 1 g/l galactose, and no added carbohydrates). C2C12 myoblasts grow more slowly without glucose irrespective of the presence of galactose, which is not consumed by the cells, and they fail to differentiate without glucose in the medium. Cells grown in a no-glucose medium (with or without galactose) have lower maximal respiration and spare respiratory capacity than cells grown in the presence of glucose. However, increasing glucose concentration above physiological levels decreases the achievable maximal respiration. C2C12 myotubes differentiated at a high glucose concentration showed higher dependency on oxidative respiration under basal conditions but had lower maximal and spare respiratory capacity when compared to cells differentiated under low glucose condition. Citrate synthase activity or mitochondrial yield were not significantly affected by changes in the available substrate concentration but a trend towards a higher respiratory chain activity was observed at reduced glucose levels.

Conclusions/Significance

Our results show that using galactose to increase oxidative metabolism may not be applicable to every cell line, and the changes in mitochondrial respiratory parameters associated with treating cells with galactose are mainly due to glucose deprivation. Moderate concentrations of glucose (1 g/l) in a growth medium are optimal for mitochondrial respiration in C2C12 cell line while supraphysiological concentrations of glucose cause mitochondrial dysfunction in C2C12 myoblasts and myotubes.     

Glucose requirement for induction by sodium butyrate of the glycoprotein hormone alpha subunit in HeLa cells

Butyric acid produces multiple effects on mammalian cells in culture, including alterations in morphology, depression of growth rate, increased histone acetylation, and modified production of various proteins and enzymes. The latter effect is exemplified by the induction in HeLa cells of the glycoprotein hormone alpha subunit by millimolar concentrations of the fatty acid. This report demonstrates that increased subunit accumulation in response to sodium butyrate is strikingly dependent on the presence of glucose (or mannose) in the growth medium. In contrast, basal levels of subunit synthesis are only marginally affected when the culture medium is supplemented with one of a variety of hexoses. An increase in the accumulation of HeLa alpha does not occur in medium containing pyruvate as the energy source, and sustained induction requires the simultaneous and continued presence of both glucose and butyrate. The effects of butyrate on HeLa cell morphology and subunit induction can be separated, since the latter is glucose-dependent while the former is not. Failure of butyrate to induce alpha in medium containing pyruvate does not result from restricted subunit secretion, since the levels of intracellular alpha are not increased disproportionately relative to those in the medium. The hexoses which support induction of HeLa alpha (glucose greater than or equal to mannose greater than galactose greater than fructose) are identical to those which have been shown previously to stimulate the glucosylation of lipid-linked oligosaccharides and enhance the synthesis of certain glycoproteins. Labeling of various glycosylation intermediates with [3H]mannose indicates that in glucose medium there is a decrease in the level of radioactivity associated with both dolicholpyrophosphoryl oligosaccharide and cellular glycoproteins and a concomitant increase in the fraction of label recovered in secreted glycoproteins. Butyrate also causes a decrease in [3H]mannose-labeled cellular glycoproteins and an increase in tritiated extracellular glycoproteins, particularly in glucose medium. Likewise, glucose stimulates the incorporation of [3H]glucosamine into immunoprecipitable alpha subunit relative to the bulk of HeLa-secreted glycoproteins, and this is further enhanced by butyrate. However, as demonstrated by lectin chromatography of conditioned media, a nonglycosylated subunit does not accumulate in pyruvate medium, either in the absence or presence of butyrate.(ABSTRACT TRUNCATED AT 400 WORDS)     

Considerations on the lactate consumption by CHO cells in the presence of galactose

A CHO cell line producing t-PA was cultured using glutamate and glucose or galactose to decrease the formation of metabolic end-products and therefore improving the process. In batch cultures using glutamate (6 mM) with glucose at two different levels (5 and 20 mM) or with glucose and galactose (5 and 20 mM, respectively) a remarkable difference in cell culture parameters was evidenced. For 20 mM glucose, a usual cell pattern was observed with lactate built-up in the medium. For 5 mM glucose, cell growth was arrested due to glucose depletion and only a limited use of the excreted lactate could be observed, not supporting cell growth sufficiently. However, when glucose 5 mM and galactose 20 mM were used together, cells consumed the glucose first and, interestingly, in a second phase they continued growing on galactose with the simultaneous consumption of the endogenous lactate. Under these conditions, cell growth was even improved with respect to growth on 20 mM glucose, used as a control. This metabolic behavior is further investigated by using metabolic flux analysis, suggesting that the lactate produced is not used in the oxidative metabolism through the TCA cycle. Metabolic fate of the lactate consumed is discussed.     

Requirement of carbohydrates for cell fusion induced by vesicular stomatitis virus

The fusion of BHK-21-KB cells by vesicular stomatitis virus was not induced in Eagle's minimal essential medium without glucose. In medium containing glucose, the rate of polykaryocyte formation decreased as the concentration of glucose was reduced below 5 mM. However, no reduction in virus production 24 hr after infection was seen under this condition. Addition of pyruvate or mannose to the culture medium caused a reversal of cell fusing activity. Cell fusion and virus growth were significantly suppressed by sodium azide and 2,4-dinitrophenol.     

Citrate metabolism in Lactobacillus casei and Lactobacillus plantarum

Information on the factors influencing citrate metabolism in lactobacilli is limited and could be useful in understanding the growth of lactobacilli in ripening cheese. Citrate was not used as an energy source by either Lactobacillus casei ATCC 393 or Lact. plantarum 1919 and did not affect the growth rate when co-metabolized with glucose or galactose. In growing cells, metabolism of citrate was minimal at pH 6 but significant at pH 4·5 and was greater in cells co-metabolizing galactose than in those co-metabolizing glucose or lactose. In non-growing cells, optimum utilization of citrate also occurred at pH 4·5 and was not increased substantially by the presence of fermentable sugars. In both growing and non-growing cells, acetate and acetoin were the major products of citrate metabolism; pyruvate was also produced by non-growing cells and was transformed to acetoin once the citrate was exhausted. Citrate was metabolized more rapidly than sugar by non-growing cells; the reverse was true of growing cells. Citrate metabolism by Lact. plantarum 1919 and Lact. casei ATCC 393 increased six- and 22-fold, respectively, when the cells were pre-grown on galactose plus citrate than when pre-grown on galactose only. This was probably due to induction of citrate lyase by growth on citrate plus sugar. These results imply that lactobacilli, if present in large enough numbers, can metabolize citrate in ripening cheese in the absence of an energy source.     

High-end pH-controlled delivery of glucose effectively suppresses lactate accumulation in CHO fed-batch cultures

A simple method for control of lactate accumulation in suspension cultures of Chinese hamster ovary (CHO) cells based on the culture's pH was developed. When glucose levels in culture reach a low level (generally below 1 mM) cells begin to take up lactic acid from the culture medium resulting in a rise in pH. A nutrient feeding method has been optimized which delivers a concentrated glucose solution triggered by rising pH. We have shown that this high-end pH-controlled delivery of glucose can dramatically reduce or eliminate the accumulation of lactate during the growth phase of a fed-batch CHO cell culture at both bench scale and large scale (2,500 L). This method has proven applicable to the majority of CHO cell lines producing monoclonal antibodies and other therapeutic proteins. Using this technology to enhance a 12-day fed-batch process that already incorporated very high initial cell densities and highly concentrated medium and feeds resulted in an approximate doubling of the final titers for eight cell lines. The increase in titer was due to additional cell growth and higher cell specific productivity.     

Glucagon regulation of pyruvate kinase in relation to phosphenol pyruvate substrate cycling in isolated rat hepatocytes.

The rate of flux through pyruvate kinase in isolated rat hepatocytes has been estimated by a new procedure involving direct spectrophotometric measurement of pyruvate production by liver cells suspended in an oxygenated medium containing lactate dehydrogenase and NADH. For the substrates, glucose, dihydroxyacetone, fructose, propionate and galactose only the rate of pyruvate production from glucose and galactose was inhibited by the addition of 1 μM-glucagon. These results imply that glucagon mediates glycolytic flux at a point in the pathway preceding the point of entry of fructose and dihydroxyacetone and not at pyruvate kinase.