SUMMARY: Kinetic studies of the effects of unlabelled acetate on CO production by a non-photosynthetic strain of C-labelled Euglena indicate the existence of a ‘labile reserve’, containing 1·5–2·0 μg.-atom C/million organisms, which is an early product of acetate assimilation and a substrate of endogenous metabolism. When this Euglena assimilated radioactive acetate, radioactivity in an ethanol-soluble form (Soluble I) increased rapidly at first, then levelled off at a value corresponding to about 0·2 μg-atom C/million organisms. Ethanol-insoluble radioactivity appeared slowly at first, then at an increasing rate which became linear at about 30–60 min. after addition of acetate. Kinetic analyses suggest that Soluble I is the precursor of the insoluble material. A second ethanol-soluble component began to become labelled some time after the first hour, and reached maximum radioactivity, corresponding to about 0·5 μg.-atom C/million organisms, 2–3 hr after addition of tracer acetate. The chemical composition of the ethanol-soluble and ethanol-insoluble components and their relationship to the ‘labile reserve’ are discussed.


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