86. Fat Metabolism in Fishes

IN previous papers in this series [e.g. Lovern, 1937] the theory has been advanced that certain fish have the ability to hydrogenate or dehydrogenate their depot fats according to their specific requirements in relation to the type of fat infested. Similar metabolic processes have been suggested for various mammals Banks & Hilditch, 1931; 1932; Hilditch, 1937; Schoenheimer & Rittenberg, 1936; Rittenberg & Schoenheimer, 1937; Longenecker, 1939]. The theory ha,s yet to be verified, however, by feeding an individual fatty acid at a high level and observing an increase in the corresponding saturated or unsaturated acid. Using eels as experimental animals, an attempt has been made to obtain such evidence by feeding certain ethyl esters. The acids chosen were myristic, palmitic and the mi-xed unsaturated acids of eel fat. Ingestion of either of the first two acids in quantity should raise the content of saturated acids in the eel's depot fat. If the theory is correct and applies to the eel, this should lead, in turn, to a compensatory dehydrogenation of some of the saturated acid, and since tetradecenoic acid is not normally an important constituent of fish fats, the probability would be that palmitic acid would be dehydrogenated. Thus feeding of either ethyl myristate or palmitate might be expected to raise the content of hexadecenoic acid in the eel's depot fat. Conversely, feeding of unsaturated esters should lead to a compensatory hydrogenation process, and since there would be little tetradecenoic acid available for this and as stearic acid (and higher saturated acids) are not usually present in other than very small quantities in fish fats, the expectation would be that hydrogenation of hexadecenoic acid would be the most important process. Such changes, if of sufficient magnitude, should be detectable by comparing the fatty acid compositions of the fats from the experimental eels and normal eels.