138 - The Cell 



organic compound is completed, it has been 

 decomposed into simple, mainly inorganic, 

 end products — and a certain quantity of 

 energy has been made available to the proto- 

 plasm. Carbohydrates and fats, as previously 

 stated, yield energy amounting to 4 and 9 

 Cal per gram, respectively, and the end 

 products of carbohydrate and fat catabolism 

 are solely carbon dioxide and water. Proteins 

 (and amino acids), like carbohydrates, yield 

 approximately 4 Cal per gram of energy, 

 when completely catabolized. The end prod- 

 ucts of protein catabolism, however, in addi- 

 tion to carbon dioxide and water, include 

 simple nitrogenous compounds, such as am- 

 monium salts and urea (from the amino 

 fractions, pp. 84 and 342), as well as sulfate 

 and phosphate salts (from the sulfur- and 

 phosphorus-containing proteins). 



Many aspects of metabolism are similar in 

 the cells of animals, plants, and other organ- 

 isms generally, and these basic patterns of 

 cellular metabolism will be considered sepa- 

 rately, in Chapter 8. Deamination reactions, 

 however, are particularly characteristic of 

 animal cells, because these cells often absorb 

 a greater quantity of amino acids than the} 1 

 need for protein synthesis. 



Deamination. The most important usage of 

 am'mo acids in each cell is for the synthesis 

 of its oivn essential protein structures. But 

 animals also derive energy from amino acids, 

 if the quantity absorbed exceeds the construc- 

 tive requirements. 



This catabolism of amino acids in animal 

 cells is achieved mainly by a series of oxida- 

 tion reactions. Before oxidation occurs, how- 

 ever, the amino fraction is split off from each 

 amino acid molecule. In man and certain 

 other higher animals, such deaminations lead 

 to the formation of urea, a very important 

 nitrogenous waste (see p. 342). But in most 

 lower animals, ammonia (or ammonium 

 salts) results from deamination, and this in- 

 organic end product becomes the chief ni- 

 trogenous waste. 



The process of deamination is not identical 

 in all cells, as is shown by the behavior of 



the enzymes (deaminases) extracted from dif- 

 ferent tissues. Frequently, however, deamina- 

 tion represents an oxidative reaction in which 

 one molecule of water participates and one 

 molecule each of ammonia (NH a ) and of 

 hydrogen peroxide (H 2 2 ) are formed as by- 

 products. This may be shown in the follow- 

 ing equation: 



CH 3 

 I 

 CH-NH, + H>0 + Oo 



I 

 COOH 



amino acid 

 (alanine) 



amino acid 

 oxidase 



CH 3 

 I 

 C=0 + NH 3 



H 2 2 



COOH 



deaminated ammonia hydrogen 

 compound peroxide 



(pyruvic 

 acid) 



Neither the peroxide nor the ammonia 

 produced in such deaminations ever accu- 

 mulates to toxic levels in the protoplasm. 

 The disposal of the peroxide as it forms is 

 effected by catalase, which catalyzes the 

 liberation of free oxygen (see p. 99). The 

 ammonia quickly is bound, forming am- 

 monium salts (for example, NH 4 HC0 3 ), 

 which are the chief nitrogenous wastes of 

 lower organisms. 



In man and other animals, another method 

 of deamination (see p. 341) is emploved, and 

 the main nitrogenous waste resulting from 

 the catabolism of the amino acids is urea 

 (p. 342). This odorless, white, crystalline 

 solid is very soluble in water, and may reach 

 fairly high concentrations in the urine, espe- 

 cially when the diet is rich in protein foods. 



Hydrolysis. Hydrolytic reactions (p. 89) 

 occur frequently in catabolism. Hydrolyses 

 are important in the mobilization of glucose 



