[Diurnal progress of NADP-linked glyceraldehyde-3-phosphate-dehydrogenase in synchronous culture of unicellular green alga Ankistrodesmus braunii and its susceptibility to X-irradiation and inhibitiors of protein synthesis].

1. The daily progress of NADP-linked GPD-activity rise in synchronous culture of Ankistrodesmus braunii was investigated in respect to short time increase of activity by light. After various exposure times cells were temporarily deprived of light and subsequently the so-called dark value as well as the light value (dark value plus light-induced part) of the enzyme activity was determined. 2. The increase of dark and light values per cell number is greater in the first half of the day than in the second. The minor activity rise in the second half seems to be caused by culture conditions since the activity of the light and dark values-after reduction of cell density to half in the early afternoon-shows a greater increase again. With regard to chlorophyll, around noon the enzymes activity reaches a maximum which corresponds to numerous other physiological maxima in synchronous algae cultures. 3. The absolute value of the light-induced part of NADP-linked GPD-activity per cell number also increases with increasing exposure time in the first half of the day more than in the second. 4. X-irradiation retards the rise of the dark value of the NADP-GPD. This is particularly evident if the cells are exposed to light for 4 hrs after X-irradiation: 10-25 krad is enough to completely arrest the rise of the dark value. 5. The light-induced part of GDP-activity is hardly affected by high X-ray doses (424 krad), either immediately following the X-irradiation altered the effect of the irradiation: the rise of the dark value was not as great as the control; the light-induced part of enzyme activity was obviously retarded more than it had been after only 4 hrs exposure time. Thus it can be assumed that with regard to the dark value of GPD-activity there is a recovery from the irradiation damage, whereas the radiation effect on the light-induced part of GDP-activity is possibly increased. 8. The D37 of chlorophyll synthesis of synchronous Ankistrodesmus cultures is approximately 85 krad and is thus, like the rise of the light-induced increase of NADP-linked GPD-activity, substantially more radiation resistent than the rise of the dark value which for a plant organism is extremely sensitive. 9. The high radiation sensitivity of the dark value rise of the GPD-activity in Ankistrodesmus braunii is compared with the relatively radiation resistent rise of this enzyme activity in resting greening Euglena gracilis, which contrary to Ankistrodesmus is not retarded by actinomycine, but only by chloramphenicol. One of the hypotheses under discussion regarding the X-ray effect is that the transcription processes which probably occur additionally for the rise in activity of the dark value of NADP-linked GPD may be the particularly radiation sensitive processes in Ankistrodesmus braunii.