Summary The technique of disc electrophoresis has been presented, including a discussion of the technical variables with special reference to the separation of protein fractions of normal human serum.

(26)(27)(28). In addition, recent work has demonstrated a further isoenzyme of malate dehydrogenase localized in plant microbodies (22,23,29). Although the isoenzymes of malate dehydrogenase in higher plant tissues have been the subject of many investigations (11,(21)(22)(23)(28)(29)(30), little is known about the distribution and function of these isoenzymes in the algal cell. When Euglena cell organelles were separated by sucrose density gradient centrifugation, malate dehydrogenase activity was recorded in several fractions (17). Besides malate dehydrogenase activity in the mitochondrial fraction, activity was present in the particulate glycolate oxidoreductase fraction, the supernatant, but not in the chloroplast fraction (17). When grown phototrophically on an appropriate light-dark regime, cultures of Euglena gracilis divide synchronously, an appropriate doubling of cell number occurring in each dark period within certain limits of cell concentration (7). By using division synchronized cultures, the development of malate dehydrogenase activity in relation to other cellular activities can be investigated. In the present paper, we report on the characterization of malate dehydrogenase isoenzymes in cell fractions and their expression over the division cycle in Euglena cultures. MATERIALS AND METHODSGrowth, Synchronization Regime, and Sampling of Culture. Division synchronized cultures of E. gracilis Klebs strain Z were obtained exactly as described previously (4). As before, samples were removed at ts, t12, t17, and ti,, this referring to the hour of sampling after commencement of the 24-hr cycle in which the culture was used. Asynchronous cultures were grown at 25 C in the photoautotrophic growth medium of Cramer and Myers (9) at a continuous light intensity of 6000 lux provided by banks of fluorescent tubes (Osram white) and gassed with air at a rate of 71/hr. Cells were harvested in middle exponential phase of growth by centrifugation at 10OOg for 4 min.CeUl

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