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Abstract
Glutamate-14C and aspartate-14C were supplied to the roots of 1-year-old MM 106 rooted apple cuttings. Both compounds were readily absorbed and translocated to the aerial parts of the plants. Glutamate, and its metabolic products, tended to accumulate in phloem tissues to approximately the same levels as were detected in the xylem. This was in contrast to aspartate, and its products, which were concentrated in the xylem.
Extensive metabolism occurred during uptake and translocation with detectable level in respiratory CO2 and in protein. However, emphasis is given to the recovery of labeled amino acids in aerial tissues. This supports previous findings indicating the significance of amino acids as N carriers in the apple.
Abstract
Glutamate synthase (L-glutamine:2-oxoglutarate aminotransferase, NADPH-oxidizing, E.C. 2.6.1.53) was isolated from bark tissue of the apple (Malus domestica Borkh. cv. Golden Delicious). The enzyme was partially purified by ammonium sulfate fractionation and diethylaminoethyl-cellulose chromatography. The enzyme was specific for L-glutamine and α-ketoglutarate as a nitrogen donor and acceptor, respectively. Both NADH and NADPH were found to be equally effective as reductants, although NADH appeared to be a physiological electron donor. The enzyme exhibited maximal activity at pH 8.0. Michaelis-Menten constants of the enzyme were 440, 50, 2.6, and 2.4 μΜ for L-glutamine, α-ketoglutarate, NADH and NADPH, respectively. L-Glutamate inhibited the enzyme activity competitively with respect to α-ketoglutarate, the estimated inhibition constant being 0.48 mm. The significance of this enzyme in the annual nitrogen recycling process in the apple is discussed.
Abstract
The extractable protein in bark tissue of 1-year-old shoots of apple trees (Malus domestica Borkh. cv. Golden Delicious on Mailing (M) 26 rootstocks) declined dramatically during spring growth, while amino acids increased. Extensive degradation of protein was first visible at the silver-tip stage of growth. Prior to this period, some redistribution of nitrogenous compounds was indicated by the increase in amino acids while protein remained unchanged. Total extractable nitrogen declined during spring growth, indicating that it was mobilized to developing tissues. In vitro activity of an acid endoprotease increased rapidly upon regrowth, but its increase preceded the in vitro decline in proteins. The autolytic activity, however, did not change in the early growth stage, and protein had already declined 50% when this activity significantly increased. Electrophoretic separation of intact proteins indicated a substantial shift in relative mobility from higher to lower molecular weights as the season advanced. However, the majority of proteins showed little evidence of net breakdown during early spring growth. Two polypeptides of 38,000 and 56,000 daltons which were present up until mid-April were not detected in extracts of shoots collected in May. The 65% decline in total protein observed by May 10, however, could not be accounted for by the loss of these 2 peptides. It is thus concluded that the immediate requirement of nitrogen at the early stage of growth (up to silver-tip) is met by the transport of soluble nitrogen present in adjacent bark and that redistributed from the wood. The large requirement of nitrogen after silver-tip stage of growth is then met by massive breakdown of storage proteins.