SOURCES OF FAT IN ANIMAL BODY 551 



Although Kossel 160 suggested that hexone bases and leucine might be 

 sugar precursors, inasmuch as they contained six carbon atoms, and Muller 

 and Seemann 161 strongly supported this hypothesis, Stiles and Lusk 162 

 presented the first experimental evidence by demonstrating the conversion 

 of a protein hydrolysate to "extra sugar" when it was given to a phlorhi- 

 zinized dog. Reilly, Nolan, and Lusk, 163 as well as Janney, 164 confirmed 

 these results. It is generally agreed that a D:N ratio of approximately 

 3.6:1 obtains in a phlorhizinized dog after the ingestion of meat (or when 

 fasting) ; this would account for a conversion of approximately 58% of pro- 

 tein to sugar. Many workers have confirmed these early classic demon- 

 strations of the conversion of protein to glycogen or to glucose. 



b'. Experiments with Amino Acids: The larger proportion of the amino 

 acids is known to be convertible to carbohydrate. Of the twenty-two 

 common amino acids, only leucine, lysine, methionine, and tryptophane 

 have been proved to be non-glycogenic, while the evidence for the end- 

 product is not clear in the case of hydroxylysine and phenylalanine. 



Friedberg and Marshall 165 summarized the experimental work carried out 

 on glycogenicity and ketogenicity of the amino acids. These workers in- 

 dicated that, since the same amino acid may give rise to pyruvate and to 

 acetate (which is convertible to acetoacetate) , the categorization of these 

 substances into "ketogenic" and "glycogenic" lacks meaning in the light 

 of present knowledge of intermediary metabolism. It is suggested that an 

 amino acid might yield both acetoacetate and glycogen but that, since these 

 are mutually antagonistic, there would be no evidence of the formation of 

 either metabolite in in vivo tests. Although this is an attractive hy- 

 pothesis, the fact remains that most amino acids are primarily glycogen 

 formers, or are convertible to acetoacetate. A list of the amino acids 

 which are definitely glycogenic is included in Table 6 (page 552). 



(b) Respiration Experiments as Proof of the Conversion of Protein to 

 Carbohydrate. The experiments employed by Williams, Riche, and Lusk 155 

 to demonstrate the change of protein to carbohydrate corresponded to 

 those previously employed in proving the formation of fat following the 

 ingestion of protein. 154 The chief difference in the technic was that, in the 

 first case, the glycogen storehouses were not previously charged before the 

 administration of the test dose of protein, while in the latter case a high 



»« A. Kossel, Deut. med. Wochschr., 24, 581-582 (1898). 



161 F. Muller and J. Seemann, Deut. med. Wochschr., 25, 209-211 (1899). 



162 P. G. Stiles and G. Lusk, Am. J. Physiol, 9, 380-385 (1903). 



183 F. H. Reilly, F. W. Nolan, and G. Lusk, Am. J. Physiol, 1, 395-410 (1898). 



164 N. W. Janney, J. Biol Chem., 20, 321-350 (1915). 



iss p Friedberg and L. M. Marshall, personal communication to the author, 1954. 



