It was shown that the development of liver glucokinase in the rat coincided with a peak in the levels of circulating thyroid hormone at about the 16th postnatal day. Administration of thyroid inhibitors blocked the development of the enzyme and administration of thyroid hormone restored activity to normal levels. Glucokinase could be induced prematurely as early as the 2nd postnatal day by the administration of thyroid hormone followed by daily injection of glucose (10 mg/g body weight). Glucocorticoids and corticotropin failed to induce glucokinase activity prematurely. The postnatal increase in circulating thyroid hormone levels together with increased intake of carbohydrate at weaning may be the normal physiological stimulus for induction of this enzyme.

No abstract

The sequence of events leading up to the establishment of symbiotic nitrogen-fixation were studied in two tropical legumes, Centrosema pubescens Benth, and Vigna unguiculata L. Walp. Parameters measured included fresh and dry weights, chlorophyll and leghaemoglobin contents, as well as the activities of NADH-nitrate reductase (EC 1.6.6.1), and nitrogenase (nitric-oxide reductase-EC 1.7.99.2) in plants that were inoculated with suitable rhizobia or which were watered with potassium nitrate. Dry weight and photosynthetic activity of both species followed the sigmoidal pattern which is characteristic of most plants. Growth was little different in either a qualitative or quantitative sense whether nitrogen was supplied as nitrate or through dinitrogen fixation. Although the biochemical sequence of events was dependent on the limiting sensitivities of the individual assays used, the data suggest that nitrate reductase is the first measurable enzymatic activity in the nodules (and roots), followed by acetylene reduction and leghaemoglobin in that order. It is possible therefore, that low levels of symbiotic nitrogen fixation occur in the nodules in the absence of leghaemoglobin. Nitrate reductase activity in C. pubescens nodules was negatively exponentially correlated with nitrogenase activity of the same nodules, suggesting a changing metabolism in old nodules. These data are discussed in terms of environmental and physical factors known to control nitrogen fixation.

No abstract