Cxvi. the Enzymic Deamidation of Asparagine

THE occurrence of asparagine in the higher plant and its role in protein metabolism have formed the subject of numerous investigations. It has been found that the amount normally present in the green plant is very small and it is only under artificial conditions, associated with a carbohydrate deficiency, that an accumulation of this substance is ever obtained. E. Schulze [1906], in a series of papers, showed that in etiolated seedlings most of the protein reserve is converted into asparagine. In the mature plant, in which the leaf is the seat of metabolic activity, it has been found since Schulze's work that, in general, starvation results in a conversion of protein to asparagine. Chibnall [1924], in the course of investigations on the nitrogenous metabolism of the higher plants, concludes that there is a constant production of asparagine from protein in the normal mature leaf, and that in plant metabolism asparagine has the role of a translocatory substance and is the chief medium whereby nitrogen, in a form suitable for subsequent resynthesis, can be conveyed from one part of the plant to another. According to Prianischnikov [1916] asparagine is formed as a temporary nitrogen reserve whenever there occurs, through some derangement of the normal course of protein metabolism, an excess of ammonia which is toxic to the plant. Important as is the part played by asparagine in protein metabolism, the reactions whereby protein and asparagine are interconverted are still far from being clearly understood. On the one side the degradation of protein through proteoses and polypeptides to amino-acids is well known, and a similar reverse process is probable; but there is little or no evidence regarding the steps intervening between these amino-acids and asparagine. It has been assumed that the intermediate forerunner of asparagine in the plant is aspartic acid, and that an enzyme, for which the name asparaginase has been used, controls the reaction Asparagine Aspartic acid + ammonia CH(NH2) .COOH CH(NH2) .COOH