SUMMARY: Glycogen and glycogen phosphorylase content of microplasmodia from were studied during carbohydrate-limited growth and spherulation, induced by starvation. The results indicate that glycogen metabolism in this organism responds most strongly to the availability of external glucose. Glycogen phosphorylase is a constitutive enzyme, present with the same specific molecular activity at all phases of growth and during transition to spherules, supporting the observation that the enzyme cannot be regulated covalently by phosphorylation-dephosphorylation. The enzyme is regulated only by metabolites, in a rather inefficient way: even during net synthesis of glycogen its degradation is not entirely stopped, resulting in a futile cycle. Its activity is merely slowed down to approximately 30% of its rate during glycogen breakdown after consumption of glucose. At this time, part of the glycogen, broken down, is apparently used for the synthesis of slime.


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