PLANT METABOLISM 



401 



of the autotrophic ^ nitrifying bacteria which oxidize ammonia to nitrite 

 {Nitrosomonas and Nitrosococcus) and nitrite to nitrate (Nitrobacter) . 

 Nitrification is an important process in the soil, but it is unreasonable 

 to assume that plants can use only nitrate, the end product of the process, 

 for their nutrition. We have already seen that they can use ammonia 

 directly, and nitrite also is an excellent source of nitrogen for plants when 

 it is present in subtoxic concentrations. Even complete ammonification 

 may be unnecessary before the nitrogen of organic residues is used. In 

 such material the nitrogen is present chiefly as protein, and upon hydroly- 

 sis this protein will yield peptides and free amino acids. For plants 

 grown in the absence of bacteria (to avoid complications from bacterial 

 action on the nitrogenous compounds) certain single amino acids can 

 serve adequately as the sole source of nitrogen. Peptone, a mixture of 

 peptides and free amino acids, also supports good growth. Evidently 

 plants can use the products at all stages of protein breakdown: peptides, 

 amino acids, ammonia, nitrites, and nitrates. 



Synthesis of amino acids 



A plant supplied nitrate or nitrite must reduce these compounds to 

 the reduction level of ammonia for the synthesis of amino acids. Whether 

 tlie reduction yields free ammonia or reduced nitrogen bound in a more 

 complex form has not been established. The best defined amino acid 

 synthesis from ammonia is the reductive amination of a-ketoglutaric 

 acid to glutamic acid by way of a-iminoglutaric acid: 



COOH COOH COOH 



I I I 



CO ^ C=NH HcNXH 



I + NH3 ^=^ I + R2O ^ ^^^ "^ > I 



(CH2)2 (CH,)2 * glut, acid (CH,), 



I I clehydrogena-ie I 



COOH COOH '^^^ COOH 



a-Ketoglutaric acid a-Iminoglutaiic acid Glutamic acid 



This reaction sequence has been accomplished with cell-free enzyme 

 preparations from plants when reduced DPN has been added. Compara- 

 ble reductive aminations may result in the formation of other amino 

 acids, although they have not been demonstrated. 



Glutamic acid is a key compound in the nitrogen metabolism of animals, 

 plants, and microorganisms. It is probable that many amino acids in 

 plants are formed by transamination from glutamic acid to various a-keto 

 acids. By transamination the amino group of an amino acid is trans- 



^ Autotrophic bacteria are those microorganisms which can derive all their energy 

 from the oxidation of a simple inorganic compound or element (e.g., oxidation of S 

 to 80^=) and are able to use carbon dioxide as their sole source of carbon. 



