Abstract
The specific activities of threo‐D~s~‐Isocitrate:NAD oxidoreductase (E.C.1.1.1.41; NAD‐ICDH) and threo‐D~s~‐Isocitrate:NADP oxidoreductase (E.C. 1.1.1.42; NADP‐ICDH) were measured in the brachial spinal cord of chick embryos ranging in development from 3.5 to 21 days' incubation. The activity of NAD‐ICDH increased gradually throughout the developmental period studied and appeared to be similar ontogenetically, with other enzymes of the citric acid cycle. The ontogenetic pattern of NADP‐ICDH, on the other hand, was quite different. The enzymatic activity increased rapidly from 3.5 to 7 days' incubation and then increased at a slower rate throughout the remainder of development. The temporal changes in activity of both enzymes are discussed in relation to other enzymes of intermediary carbohydrate metabolism and to the processes of neuroblastic growth and differentiation. The results of this study suggest that NADP‐ICDH is the major source of reduced NADP for the reductive syntheses associated with neuroblastic growth and differentiation whereas glucose‐6‐phosphate dehydrogenase and the pentose cycle provide the major source of NADPH during the preceding period of rapid proliferation and early differentiation.