Development and intracellular distribution of enzymes of the oxidative pentose phosphate cycle in radish cotyledons

Abstract

Developmental and compartmentation studies were used to evaluate the relative roles of the oxidative pentose phosphate cycle, the Calvin cycle, and the glycolysis in cotyledons of radish (Raphanus sativus L.).Glucose-6-P dehydrogenase, 6-P-gluconate dehydrogenase, glucose-6-P isomerase, and the NAD-dependent glyceraldehyde-3-P dehydrogenase were present in high activity in ungerminated seeds, increased about 2-fold during germination in the dark, and were slightly enhanced by light. In contrast, NADP-dependent glyceraldehyde-3-P dehydrogenase was developed to only a small degree in the dark, but increased severalfold in continuous white or far red light. The activity of phosphofructokinase was low throughout germination.The separation of cell compartment-specific isoenzymes showed that, except in ungerminated seeds, the plastid enzyme accounted for 40 to 45% of the total activity of 6-P-gluconate dehydrogenase and for 15 to 20% of glucose-6-P isomerase. The remaining activity was due to the cytosolic isoenzymes. The presence of glucose-6-P dehydrogenase and 6-P-gluconate dehydrogenase in plastids was also established by their presence in the isolated organelle. The NAD-dependent glyceraldehyde-3-P dehydrogenase was mostly due to the cytosolic isoenzyme, whereas the NAD-dependent activity associated with the NADP-dependent glyceraldehyde-3-P dehydrogenase was very small.The data indicate that the enzymes of the oxidative pentose phosphate cycle are present in the cytosol throughout germination. In the plastids these enzymes already became fully developed during early germination in the dark, whereas enzymes of the Calvin cycle increased only in the light. Glycolysis seemed to be of minor importance.