114 



THE CELL AND PROTOPLASM 



the energy transfer during anabolism has 

 come to occupy an increasingly important 

 place. 



The most appealing concept of such a 

 mechanism so far expressed is the one in 

 which the catabolic process leads to the 

 formation of products from which a direct 

 synthesis of building stones of the cell con- 

 stituents would be possible by means of 

 (thermodynamically) spontaneous reactions 

 (Kluyver and Donker 1926 ; Kluyver 1931 ; 

 Knoop 1931). Experimental evidence in 

 favor of this hypothesis is certainly not 

 lacking. In the first place one might 

 point out that among the typically cata- 

 bolic processes synthetic reactions are like- 

 wise far from rare. Some specific ex- 

 amples are the formation of 4-carbon from 

 2-carbon or 3-carbon compounds, such as 

 acetylmethyl carbinol from acetaldehyde 

 or from acetic acid (Reynolds and "Werk- 

 man 1937) and of succinic acid, presum- 

 ably from a 3-carbon acid and carbon diox- 

 ide (Wood and Werkman 1936; Elsden 

 1938). More striking is the production of 

 large quantities of caproic acid from ethyl 

 alcohol during the methane fermentation 

 of the latter substance (Barker 1937). 

 The importance of this conversion is that 

 it shows how readily a synthesis of a 2- 

 carbon to a 6-carbon compound can be 

 achieved as a result of catabolic activity. 



Long before this demonstration the for- 

 mation of fatty acids as a result of con- 

 densations of a 2-carbon compounds had 

 been proposed, mainly based upon the 

 studies by Hahn and Kinttof (1925) on 

 the formation of fats by the yeast Endo- 

 myces vernalis. The recent chemical syn- 

 thesis of stearic acid at low temperatures 

 by condensation of crotonic aldehyde and 

 reduction of the resulting unsaturated 

 aldehyde (Kuhn, Grundmann and Trisch- 

 mann 1937) lends considerable weight to 

 the idea that fatty acids do originate 

 through a condensation process of alde- 

 hydes. Reichel and Schmid (1939) have 

 added the important observations that 

 Endomyces vernalis is capable of produc- 

 ing condensation products only from un- 

 saturated aldehydes ; the saturated ones are 



converted into the corresponding fatty 

 acids. With a mechanism available by 

 which unsaturated higher aldehydes can 

 be formed, the synthesis of fatty acids 

 would, then, not offer serious difficulties. 

 The mechanism for this production would 

 thus be the gradual breakdown of the or- 

 ganic substrate which is being attacked in 

 catabolism, with the formation of inter- 

 mediate products from which a direct 

 spontaneous synthesis is possible. 



The spontaneous synthesis of amino acids 

 on an equal basis from keto- or unsatu- 

 rated acids and ammonia is an established 

 fact. It has been shown to occur chem- 

 ically (Knoop and Oesterlein 1925, 1927) 

 as well as biochemically, under the influ- 

 ence of enzyme preparations from Pseudo- 

 monas fliiorescens (Virtanen and Tarnanen 

 1932) and of yeast (Haehn and Leopold 

 1937). 



These examples may show that in a few 

 cases there is not necessarily a qualitative 

 difference between anabolic and catabolic 

 reactions in the sense that only the latter 

 would occur as thermodynamically spon- 

 taneous reactions. Given the proper initial 

 substances, these anabolic, synthetic proc- 

 esses also take place spontaneously. And 

 one is thus led to consider the possibility 

 that catabolic processes are not as impor- 

 tant in supplying the energy for various 

 vital activities as they are in furnishing 

 the raw materials from which the assimila- 

 tory processes take place as "modified" 

 catabolic reactions. Such a concept is sup- 

 ported remarkably well by a number of 

 recent studies. 



In the first place, a theoretical conse- 

 quence of this point of view would be the 

 existence of a stoichiometric rather than 

 of an energetic relationship between the 

 possible synthetic and the breakdowTi reac- 

 tions. For if a substrate were available to 

 assimilatory reactions only after it had 

 given rise to e.g., acetaldehyde, then the 

 maximum possible assimilation would de- 

 pend upon the maximum producible alde- 

 hyde, rather than upon the amount of 

 energy liberated during the conversion of 

 the substrate into acetaldehyde. 



