Metabolic relations of urease in Citrullus vulgaris Schrad

Abstract

The enzyme urease is present in considerable quantity in the cotyledons of Citrullus seedlings, though elsewhere in the plant it is present only in traces or is absent; this has been shown by qualitative, quantitative and histo-chemical methods. The present investigation is aimed at elucidating the possible metabolic role of cotyledonary urease. The enzyme concentration has been found to fluctuate during the course of growth of the cotyledon, showing an initial rise followed by a sudden drop to practically zero. It has been previously suggested that the urease may be no more than a reserve protein. However, experiments conducted to verify this hypothesis have shown that the protein level in the cotyledons remains practically constant at the time of urease disappearance. Another suggested possibility that the urease in the cotyledon is connected with urea metabolism of the seedling - has also been found to be invalid. A detailed examination of the biochemical changes in the cotyledon during germination has been made and the possibilities of correlation of urease activity with chlorophyll content, water content, respiration and cell extension have been examined. Experiments were also conducted to ascertain whether the urease activity is affected by a change in nutritional status. These have shown that the cycle of urease change is independent of external nitrogen supply but is affected by the addition of glucose, which brings about an early drop in urease activity; the possible reason for this interaction is discussed. Lastly, the changes in urease activity in detached cotyledons have been studied under different experimental conditions. In general, all the experimental results suggest that urease metabolism is connected solely with the growth and development of the cotyledon; and the very close correlation between cell extension and urease change suggests that the disappearance of urease in the cotyledon is simply one aspect of the "protoplasmic differentiation" that takes place as the cell extends. The high urease content of the young cotyledon is thus probably merely an indication of the state of the immature protoplasm of the unextended cells; there is no evidence that it is concerned in the metabolism of urea.