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Notes: Summary 1. The increase in the activity of the NADP+-dependent glyceraldehyde-3-phosphate dehydrogenase by irradiance and its reversal by darkness were measured. A T-values (activity after irradiance: activity in the dark after complete loss of activation) were plotted against increasing time of irradiation as well as against increasing time of darkening. 2. The A T-values for irradiance as well as for darkening yield sigmoidal types of curves, and are similar to those corresponding to the integral of a curve of the type y=e -x 2(Gauss-curve). The (theoretical) sigmoidal curve was transformed (by double variable transformation) to optimally cover the experimental data. By differentiation Gauss-curves are obtained which express the change of enzyme activity per unit of time. The maximal points of these curves correspond to the half maximum of activation (not activity!) and to the highest change of activity per unit of time. 3. Figs. 3, 4 and 5 show that the increase in activity after onset of irradiation starts earlier than does the decrease after onset of darkening. Conditions which may cause the preservation of the activated enzyme during about 100 sec after darkening are discussed. However, the decrease is much more rapid later on than is the increase in the light; the change in activity (change of A T) per unit of time is smaller in the activation process than it is in the inactivation process. Thus the Gauss-curve becomes much sharper in the latter case. 4. Constant irradiance (irradiation with constant E eduring a given time period) at different points of dark-inactivation causes an activation velocity which increases with the time of the preceding dark period (Fig. 6). Therefore the Gauss-curve becomes sharper with increasing time of the preceding dark period (Fig. 7). 5. Increasing irradiance (irradiation with constant E eduring an increasing time period) at 3 different points of inactivation by variation of the time of predarkening also results in an increase in activation velocity with increase of time of predarkening (Figs. 8 and 9). 6. The model which best fits earlier experimental results as well as the kinetic studies is one in which activation is accomplished by association of four subunits of the enzyme (E o) to the activated tetrameric form (E akt). NADPH, created during the non-cyclic electron flow, could be the allosteric effector. Thus the activation velocity (change in activity per unit of time) is dependent on the concentration of E o. Both increase with increasing time of darkness. In contrast the time period of activation at constant irradiation is independent of the concentration of E o and is only dependent on the amount of the effector (NADPH), which increases with increasing time (increasing irradiance).

Notes: Summary 1. The relation between light intensity and the ratio light activity: dark activity of NADP+-dependent glyceraldehyde-3-phosphate-dehydrogenase (as a measure of the increase in activity caused by light) is expressed by a saturation curve. It is therefore similar to the curve expressing the relationship between light intensity and photosynthesis. 2. Very high light intensities not only diminish the degree of enzyme activation by light, but reduce the enzyme activity below the dark value. 3. The increase of enzyme activity is effected not only by red (maximum intensity 671 nm) but also by blue (450 nm) and green (525 nm) light. At the same quantum density blue light is more effective than green and red, which have the same effect. At the same light intensity green light shows the smallest effect, while blue light has the greatest effect. 4. It is assumed that chlorophyll is the responsible receptor pigment involved in the increase in activity of the NADP+-dependent glyceraldehyde-3-phosphate-dehydrogenase caused by light.