NITROGENOUS METABOLISM OF PYRUS MALUS L.: II. THE DISTRIBUTION OF NITROGEN IN THE INSOLUBLE CYTOPLASMIC PROTEINS

Before undertaking the experimental work on the distribution, throughout a year's cycle, of the soluble nitrogenous constituents of the leaves and branch growth, which will be presented in the third paper, it was considered of interest to obtain information respecting the nature of the insoluble leaf proteins and to ascertain what fluctuations, if any, occur during development, in the distribution of its nitrogen groups (i.e., its amino acid makeup). An additional motive for a closer study of this insoluble portion was afforded by the relationships which NIGHTINGALE (8) seemed to find between his alcohol insoluble nitrogen fraction, and carbohydrates, in the vegetative and reproductive processes of the tomato plant (Lycopersicum esculentum). Sufficient knowledge is not yet available to enable these insoluble proteins to be isolated in a state of purity, but VAN SLYKE (11, 12) has devised a method for determining the distribution of nitrogen in the various groups of the protein molecule which has furnished means of classifying different proteins by their amino acid make-up. This method was, however, devised for pure proteins and their hydrolytic products (1, 2, 3), but as MORROW and GORTNER (7) have pointed out, the extension of the method may be made to impure proteinaceous substances when due precautions are taken and no attempt is made to assign any part of the nitrogen to any specific amino acid. Experimental Samples of leaves were collected, from a 15-year-old Stayman Winesap tree growing in sod, at three critical physiological periods, viz. (1) at the early period of bud formation (May 13), (2) at the stage of active maximum growth (July 22), and (3) at the period of chlorophyl degeneration, i.e., of declining metabolic activity (Nov. 11). Five grams of each residuum, dried in vactco, remaining after the extraction with water as described in the preceding paper and consisting, therefore, of the insoluble cytoplasmic proteins and the cellulose material of the cell wall, together with such non-protein nitrogenous materials as fats, lecithins, chlorophyl, and the purine and pyrimidine bases, were ex-