| Abstrakt: |
Amounts of various free amino acids, as well as of other nitrogenous compounds, have been determined in leaves of Kikuyu grass (Pennisetum clandestinum, Hockst) and Algerian oats (Avena sterilis, L) starved in atmospheres of air and of nitrogen. Respiration rate and carbohydrate contents were also determined. Examination of nitrogen balance sheets indicates that in Kikuyu leaves starved in air transformations of nitrogen compounds are accounted for by hydrolysis of proteins to amino acids and subsequent oxidation of these to asparagine and ammonia. Whilst these processes also occur in oats, the latter differs from Kikuyu in the formation of undetermined nitrogenous compounds during late stages of starvation; in accumulation of peptides; more extensive breakdown of protein; and in secondary formation of glutamine. During starvation in air, amino acids are preferentially oxidized, the order of utilization of those determined being cystine, glutamic acid, arginine, tyrosine and tryptophane. During starvation in nitrogen very little breakdown of protein and accumulation of amino acids occurs unless the leaves are injured. Amino acids only accumulate and no asparagine or ammonia is formed. The data of other papers by the authors, and those of other workers are investigated in the light of the present findings and the following conclusions are drawn as to the nature of relations between proteins and amino acids in leaves: (a) the presence of a pattern in which proteins, pigments, phosphatides and ascorbic acid are interlocked components, provides steric hindrance which prevents protein hydrolysis in nitrogen. In air all components of the pattern are subject to oxidation to a degree depending upon the rate of carbohydrate oxidation; the dynamic character of the pattern in air allows enzymes to come into contact with their protein substrate. (b) the relations between protein and total amino acid concentration can be expressed by a curve concave to the amino acid concentration axis; at least part of the concavity results from the observed facts that some amino acids are oxidized more rapidly than others. A reversible relation between protein and amino acids is implied, modified by preferential oxidation of some amino acids. (c) the curve relating protein and amino acid content is not directly affected by respiration rate; but observations of various workers under widely different experimental conditions are all in harmony with the view that protein content is determined by the rate at which carbohydrates are utilized in an oxidation cycle. It is suggested that maintenance of a definite protein level depends upon whether syntheses of the most readily oxidized amino acids from carbohydrates and nitrogenous substrates take place at a rate at least equal to their rates of oxidation. [ABSTRACT FROM AUTHOR] |
Plný text