Abstract
Abstract 1. A differential incorporation of 14C-glucose into two distinct molecular weight fractions of glycogen from the tapeworm Hymenolepis diminuta has been observed. Changes in these labeling patterns as a function of the glycogen level have been examined. 2. After incubation of Hymenolepis diminuta in media containing 14C-glucose under steady state conditions of glycogen metabolism, the specific activity of the high molecular weight fraction was significantly greater than that of the low molecular weight fraction. This difference was due to an almost 3-fold greater specific activity in the phosphorylase limit dextrin of the high molecular weight fraction. 3. By contrast, upon reduction of the glycogen level from 8% to approximately 1.5% and subsequent resynthesis of glycogen from 14C-glucose, the specific activity of the high molecular weight fraction was much lower than that of the low molecular weight fraction. 4. The relative rates of incorporation of 14C-glucose into glycogen as a function of the initial glycogen level were determined. 5. In any given glycogen sample, regardless of the glycogen level, the high and low molecular weight fractions had identical outer chain lengths and the same degree of branching. 6. The results obtained establish significant metabolic differences between the high and low molecular weight glycogen fractions.
Highlights
While a relatively large amount of glucose was utilized by the worm under these conditions, little or no net increase in the glycogen concentration was observed
Since the percentage of the glucose taken up which was converted to glycogen for any period of incubation remained unchanged, the rate of glycogen turnover remained virtually constant (Table II)
The specific activities of the limit dextrins of the high molecular weight fractions were 2.3 to 3.3 times greater than those of the low molecular weight limit dextrins (Table III)
Summary
Glycogen was isolated by the cold water extraction procedure described previously [3], except that 0.2 M glycine buffer (pH 10.7) was used throughout. Sedimentation coefficient distribution diagrams were determined as described previously [4]. High and low molecular weight fractions of glycogen were separated by differential zone centrifugation in a density gradient of aqueous lithium bromide solution varying in density from 1.05 to 1.20 at a temperature of 20”. Samples (0.7 ml at a glycogen concentration of 2%) were layered on each gradient (total volume, 25 ml) and centrifuged at 40,000 x g in the Spinco bucket rotor, model SW 25.1, for from 15 to 45 min, depending on the sedimentation coefficients of the components to be separated. Fractions were recovered by removal of an upper portion of the gradient column (containing the low molecular weight fraction) and subsequent isolation from the lithium bromide solutions by precipitation with ethanol. The purity of each fraction obtained was verified by ultracentrifugal analysis in the model E Spinco analytical ultracentrifuge
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