Effects of adenosine 3',5'-monophosphate and adenosine 5'-monophosphate on glycogen degradation and synthesis in Dictyostelium discoideum

Glycogen phosphorylase from Neurospora crassa: purification of a high-specific-activity, non-phosphorylated form

A highly active glycogen phosphorylase was purified from Neurospora crassa by polyethylene glycol fractionation at pH 6.16 combined with standard techniques (chromatography and salt fractionation). The final preparation had a specific activity of 65 +/- 5 U/mg of protein (synthetic direction, pH 6.1, 30 degrees C) and was homogeneous by the criteria of gel electrophoresis, amino-terminal analysis, gel filtration, and double immunodiffusion in two dimensions. The enzyme had a native molecular weight of 180,000 +/- 10,000 (by calibrated gel filtration and gel electrophoresis) and a subunit molecular weight of 90,000 +/- 5,000 (by sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Each subunit contained one molecule of pyridoxal phosphate. No phosphoserine or phosphothreonine was detected by amino acid analysis optimized for phosphoamino acid detection. The enzyme isolated from cells grown on high-specific-activity 32Pi (as sole source of phosphorus) contained one atom of 32P per subunit. All the radioactivity was removed by procedures that removed pyridoxal phosphate. Thus, the enzyme could not be classified as an a type (phosphorylated, active in the absence of a cofactor) or as a b type (non-phosphorylated, inactive in the absence of a cofactor). The level of phosphorylase was markedly increased in mycelium taken from older cultures in which the carbon source (glucose or sucrose) had been depleted. The polyethylene glycol fractionation scheme applied at pH 7.5 to mycelial extracts of younger cultures (taken before depletion of the sugar) resulted in co-purification of glycogen phosphorylase and glycogen synthetase.

Fourth expansion and glucose perturbation of the Dictyostelium kinetic model

The kinetic model of carbohydrate metabolism has been expanded to include: (a) the accumulation of alpha and beta-cellulose, insoluble cell-wall glycogen and mucopolysaccharide; (b) the role of RNA turnover as a source of carbon for end-product synthesis and as a buffer regulating the level of uridine nucleotides in this metabolic network; and (c) the role of purine-nucleoside phosphorylase, 5'-AMP nucleotidase, nucleosidediphosphate kinase and polynucleotide phosphorylase. One of many predictions based on this model is that cells differentiating in the presence of glucose will produce sorocarps with an abnormally high trehalose to cellulose ratio. External perturbation of either the model or of developing cells by glucose increases the levels of sorocarp trehalose and glycogen, 5-fold and 6-fold respectively. Evaluation of the experimental data and the simulation analyses have allowed several predictions to be made concerning the compartmentation of metabolites and the permeability of cells to glucose during differentiation.

Localization of glycogen synthetase during differentiation of presumptive cell types in Dictyostelium discoideum

Ultramicrochemical techniques were utilized to assay glycogen synthetase (EC 2.4.1.11) activity in cell samples of Dictyostelium discoideum as small as 0.01 mug (dry weight) in reaction volumes of 0.1 mul. The activity was assayed by an amplification procedure employing the enzymatic cycling of pyridine nucleotides. These techniques were used to determine the extent of localization of glycogen synthetase in the two cell types during differentiation of D. discoideum. Localization studies in developing spore cells revealed decreasing enzyme activity to the culmination stage. During this phase of development, the enzyme required the presence of soluble glycogen for activity. From culmination to sorocarp stage, enzyme activity increased and was independent of the soluble glycogen. In developing stalk cells, synthetase showed a decreasing gradient of activity. In sorocarps, the cells in the stalk apex showed synthetase activity similar to that of the spores. The cells at the bottom of the stalk had no detectable activity.

Inhibition of development in Dictyostelium discoideum by sugars

Sugars such as glucose, maltose, and trehalose, which are metabolized by Dictyostelium discoideum and which enhance vegetative growth, inhibit the development of the slime mold at concentrations which stimulate growth maximally. They block the acquisition of aggregation competence as well as aggregation. The same sugars also inhibit the degradation of preformed glycogen ribonucleic acid, and protein, which is characteristic of development and which occurs when the amoebas are starved by incubation in dilute phosphate buffer.

Rates of accumulation of glycosidase activities during growth and differentiation of Dictyostelium discoideum

1. The rates of accumulation (enzyme units/h per 10(8) cells) of a number of glycosidase activities were studied in Dictyostelium discoideum cells during the growth and differentiation phases of this organism's life cycle. 2. The rates of accumulation of the enzymes beta-N-acetylglucosaminidase, alpha-glucosidase and beta-galactosidase remain unchanged during the growth and early differentiation phases. 3. The considerable changes in specific activity of the enzymes which occur in the early differentiation phase are due to the massive loss of total cellular protein which occurs at this time. 4. Significant alterations can occur in the rates of accumulation of alpha-mannosidase during both the growth and differentiation phases, and since, on the onset of differentiation, beta-glucosidase activity is excreted and degraded, the rate of accumulation of this enzyme differs in the growth and differentiation phases. 5. The characteristic rates of accumulation of all these glycosidases change markedly with changes in the growth conditions of the myxamoebae, and thus these rates of synthesis must be regulated independently; however, addition of cyclic AMP to the growth medium has no effect on them.