The Influence of Temperature on the Nature of the Fat formed by Living Organisms

LEATHES and RAPER [1925, p. 119] have put forward the view that the tem- perature at which fats are formed in nature is one of the factors which determine their degree of unsaturation. This suggestion is based mainly on three lines of reasoning. Firstly, a general survey of the fats found in animals and plants indicates that the highly unsaturated fats are most commonly found in plants growing in temperate or cold regions and in the poikilothermic animals which live under relatively cold conditions, whilst the most saturated fats are found in tropical or subtropical plants and in warm-blooded animals. The carnivora should not be included in this generalisation because the composition of their adipose tissue fat is partly, at any rate, determined by the fat which they take as food, and even in animals such as the pig, which produce fat easily from carbohydrate, differences have been observed in the composition of their stored fat which may be referred to differences in the composition of the fats present in the cereals on which they have been fed. Secondly, it is usual to find that the fat present in living organisms is fluid at the temperature at which the organisms exist. This would preclude the formation of saturated acids to any great extent in organisms living under temperate or cold conditions because such acids would tend to raise the melting points even of mixed glycerides which contained them. The higher melting point of the saturated acids would be less important from this point of view in the warm-blooded animals or tropical plants and it might be more likely therefore to expect saturated acids to occur to a greater extent in organisms living under warmer conditions. Thirdly, various hypotheses have been made regarding the chemical reactions by which fatty acids are formed in nature, viz. autocondensation of acetaldehyde [Leathes, 1906], condensation of pyruvic acid and acetaldehyde [Smedley and Lubrzynska, 1913], or autocondensation of pyruvic acid All these admit of the possibility of the formation of long chains of carbon atoms with unsaturated linkages as pro- ducts of the condensation reactions which produce them and if they are to give rise to saturated carbon chains a reduction must take place subsequently.