A STUDY WITH ISOTOPIC NITROGEN OF PROTEIN METABOLISM IN DETACHED RUNNER‐BEAN LEAVES

Abstract

The concept of Gregory & Sen (I937) that a protein cycle is continually at work in plant cells has received strong support from experiments with isotopic nitrogen. The pioneer work of Schoenheimer and his colleagues with mature animals in nitrogen equilibrium showed that dietary nitrogen containing a known excess of the isotope 15N, when fed in the form of ammonium salts or amino-acids, became rapidly incorporated in the oc-amino group of many different amino-acid residues of the tissue proteins (cf. Schoenheimer & Rittenberg, 1940). The protein of buckwheat leaves underwent a similar modification when the young plants were grown for 47 hr. on a culture solution in which the ordinary ammonium chloride was replaced by a sample containing I12 atom %0 excess '5N. Little growth occurred during this short period, and Vickery, Pucher, Schoenheimer & Rittenberg (I939, I940) concluded that not less than 6% of the protein nitrogen of the leaves must have undergone replacement by interaction with nitrogen from simpler substances in the leaf. These same workers made a more extensive investigation with a young tobacco plant. After 72 hr. on a similar culture solution the ammonia absorbed had been rapidly assimilated into the nitrogen of amides and amino-acids and into the proteins. Much of the observed chemical change could be correlated with growth, but the quantity of isotope found in the tissue proteins was appreciably in excess of that to be expected from growth alone. The excess was interpreted as the result of a continuous chemical interaction between the tissue constituents and the nutrient, during the course of which the isotopic nitrogen became introduced into the proteins. This was held to furnish an example of the normal process of nitrogen assimilation. As these workers pointed out, the introduction of isotopic nitrogen into the proteins follows as a logical consequence if a protein cycle is operative. Parallel experiments by Hevesy, Linderstrom-Lang, Keston & Olsen (I940), with sunflower plants cultured for I2 days on ammonium sulphate containing 2-5 atoms %0 excess '5N, showed that the old leaves, in which no apparent growth took place, had replaced 120% of the protein nitrogen. In the present work the problem has been investigated from a different point of view. As mentioned in the preceding paper (Chibnall, I953), the concept of a protein cycle continually at work in leaves would suggest that the rapid loss of protein observed when they are detached from the plant is due to some impairment in the mechanism of protein synthesis, so that protein breakdown predominated. Though masked, protein synthesis would thus still be going on, and it should be possible to demonstrate this by feeding the detached leaves via the petiole with a nutrient solution containing isotopic nitrogen in some convenient form.