The Metabolism of the Organic Acids of Tobacco Leaves. Xx. Effect of Culture of Excised Leaves in Solutions of Potassium Aspartate and of Asparagine.

Since the advent of paper chromatographic methods in 1944 (l), extracts of a great many plant species have been examined for the presence of amino acids. Aspartic acid and its amide asparagine have almost invariably been found. Recognition of the importance of these substances goes back to the earliest days of plant biochemistry. Vauquelin and Robiquet first isolated asparagine in 1806 (2) from asparagus sprouts, and Plisson prepared aspartic acid from it in 1827 (3). Schulze devoted much of his long lie to the investigation of the metabolism of asparagine in seedlings of lupines, and in 1898 (4) proposed the first acceptable general hypothesis of amide metabolism, later extended by Prianischnikow (5), and still regarded as essentially valid. According to this view, the protein stored in the cotyledons of the seeds undergoes complete hydrolysis during sprouting. The amino acids are then oxidatively deaminated and the ammonia produced combines with a non-nitrogenous substance, today commonly assumed to be oxaloacetic acid, with the formation of aspartic acid which, in turn, condenses with a second molecule of ammonia to form asparagine. Schulze held that the function of asparagine in the lupine seedling is to serve as the means whereby nitrogen is transported from the cotyledons to the cells of the rapidly growing tissues of the sprout where it is used for the synthesis of protein. That asparagine accumulates when detached plant leaves are kept in darkness with their petioles in water has been known since the classical work of Borodin in 1878 (6), and the phenomenon has been extensively studied since that time. Chibnall reviewed the situation as it existed in 1938 in his Silliman lectures at Yale University (7)) showing that the accumulation occurs at the expense of the protein nitrogen, and that the necessary oxaloacetic acid presumably arises through the agency of the tricarboxylic acid cycle. Notwithstanding a great deal of subsequent investigation in many laboratories, there still remains much to be explained. For example, in a study of the chemical changes that occur when Connecticut Shade-grown tobacco leaves are subjected to the operation of curing by being hung in a darkened barn (8), of the 2.27 g of protein nitrogen per kg of fresh weight present initially, 1.26 g had disappeared as such after 8 days. Of this nitrogen, 71 y0 was then present combined as asparagine, 12% as ammonia, and 4.6% as glutamine. Only 4% remained as the amino nitrogen of amino acids other than the two amides. Frankenburg et al. (9) in a somewhat similar study of the curing of Connecticut Shade-grown leaves found that, after 8 days, asparagine and