230 PLANT BIOCHEMISTRY 



Oleic and linoleic acids together total about 80 per cent of the fatty 

 acids of the seed fats of the world. Palmitic acid, next most abundant, 

 may amount to 10 per cent, and all the others combined make up the 

 remaining 10 per cent. Obviously the biosynthesis of these three most 

 common acids is of first importance. Yet some seeds are extremely 

 high in other fatty acids. For example, nutmegs produce 75 to 80 

 per cent of their fatty acids as myristic acid, palm species 50 per cent 

 lauric and 20 per cent myristic acids, and seeds of Cruciferae 30 to 40 

 per cent erucic acid; in castor beans ricinoleic acid reaches 90 per 

 cent of the total fatty acids. Thus the fats are to a large extent charac- 

 teristic of the species concerned. Perhaps the various species possess 

 the same or quite similar mechanisms for their biosynthesis, but if so 

 they must control these mechanisms in quantitatively different ways 

 to form such diverse products. 



Carrying this matter of differences a step farther, there is evidence 

 that flax, among other plants, forms oils of varying composition 

 depending upon climatic location. The same major fatty acids are 

 probably always present, but their proportions change. Moreover, it 

 is clear that the proportions of fatty acids change during the maturing 

 of fruits and seeds, indicating a change in the control process during 

 ripening. Cottonseed, 31 days after flowering, contains 2.4 per cent 

 lipides with 23.9 per cent saturated, 29.3 per cent oleic, and 46.8 

 per cent linoleic acids. After 29 days longer the total lipides reach 

 25 per cent with 22.4 per cent saturated, 25.5 per cent oleic, and 

 52.1 per cent linoleic acids. Such changes are probably typical and are 

 much exceeded in soybeans and flaxseed. 



A decrease in the percentage of components was taken for a time 

 to mean that the fatty acids showing the decreasing percentages were 

 being converted into those of increased percentage. It turns out that 

 the saturated acids and oleic acid are relatively abundant in young 

 seeds and decrease proportionately in the mature forms. This observa- 

 tion led to the theory that the saturated fatty acids are synthesized 

 first and later dehydrogenated. Now, however, the discovery that all 

 appear to increase in absolute amount, though at different rates 

 during ripening of the seed, suggests that the saturated fatty acids are 

 not precursors of unsaturated types. Instead different routes of syn- 

 thesis have been suggested with changes in the rates with age to 

 account for the observed changes in the proportions of fatty acids. 



Castor beans and flaxseed utilize radioactive acetate in the synthesis 

 of saturated fatty acids. The mechanism is speculated to be like the 

 multiple condensation of acyl coenzyme A outlined on page 458 for 



