664 



NATURE 



[July 21, 1921 



Quality of Protein in Nutrition.^ 

 By Dr. R. H. A. Plimmer. 



THE normal diet of man and animals contains 

 certain nutritional elements every one of which 

 is essential for the maintenance of life and health. 

 These elements are : 



(1) Proteins, complex nitrogenous substances found 

 in meat, milk, eggs, cereals, and plant tissues. 



(2) Carbohydrates, such as starch in cereals, sugars 

 in fruits, milk, etc. 



(3) Fats, such as butter, lard, suet, and vegetable 

 oils. 



(4) Salts, or the mineral constituents in meat, milk, 

 cereals, vegetables, etc. 



(5) Vitamin A, contained in butter, cod-liver oil, 

 eggs, green vegetables, etc. 



(6) Vitamin B, contained in yeast, germ of cereals, 

 meat, eggs, etc. 



(7) Vitamin C, contained in some fruits and some 

 vegetables. 



(8) Water. 



If w^e examine these food elements in fuller detail 

 we find that in whatever form the carbohydrate is 

 taken in the food it is converted during digestion 

 in the body into a simple sugar, such as grape-sugar; 

 so that for nutritional purposes all carbohydrates can 

 be considered the same. They are burnt up like coal 

 to supply the body with heat and energy. 



Fat of almost every source consists mainly of three 

 triglycerides, palmitin, olein, and stearin. The con- 

 sistency of fats depends simply on the relative pro- 

 portions of these substances. Certain fats are the 

 vehicles of the A vitamin, but, leaving the vitamin 

 out of consideration, fats are of equal value in nutri- 

 tion, and, like carbohyflrates, they supply fuel for 

 heat and energy. Fats can be built up in the body 

 from the carbohydrate in the food. Some very recent 

 feeding experiments by Osborne and Mendel indicate 

 that fat, as such, can be omitted from the diet if the 

 vitamin A is supplied in a specially prepared form. 

 The special value of fat in nutrition thus depends on 

 the A vitamin associated with it, and not on its 

 chemical constitution. 



The mineral salts in an ordinary mixed diet do 

 not need to be supplemented, but generally some 

 sodium chloride is added. Animals on cereal diets 

 must be supplied with this common salt. 



Whatever is the source of the three vitamins, so 

 far as we know the A vitamin is the same whether 

 it be in butter or cod-liver oil, B vitamin is the same 

 in yeast and cereal germ, and C vitamin the same 

 in orange-juice or cabbage. 



Thus, since each of these elements of the diet is 

 reduced to a simple common basis during digestion, 

 we cannot speak of quality of carbohydrate, fat, or 

 vitamins. 



The protein constituent differs from all the others 

 by its endless variety. This is obvious to the naked 

 eye. For instance, the protein in white of egg is 

 in solution, and sets to a hard mass on boiling. 

 Meat protein is already in a solid form. Milk con- 

 tains two kinds of protein, the casein which is used 

 to make cheese and an albumin like egg-albumin in 

 the whey. The presence of protein in cereals is 

 scarcely recognised, as it Is obscured by the large 

 amount of starch, yet about one-tenth of flour Is pro- 

 tein ; In fact, two very special proteins are present, 

 the one soluble In alcohol, the other insoluble 

 but soluble In dilute alkali. 



^ From a discourse delivered at the Royal Institution on Friday, April 8. 



NO. 2699, VOL. 107] 



Our usual classification of proteins already indicates 

 their differences, but the variety is really far greater. 

 We need only refer to their chemical analysis. 

 Fischer, Kossel, and their pupils have shown that 

 proteins on hydrolysis break down into some eighteen 

 or twenty amino-acids. These numerous units can be 

 arranged for convenience into eight groups : 



(i) Simple M ono-amino- Acids : Glycine, alanine^ 

 valine, leucine, and tsoleucine 



(2) Mono-amino-Dibasic Acids : Aspartic acid and 

 glutamic acid. 



(3) Hydraxyamino-Acids : Serine and hydroxy- 

 glutamic acid. 



(4) Heterocyclic Acids ; Proline and hydroxy- 

 proline. 



{^) M ono-amino- Acids with Aromatic Nucleus: 

 Phenylalanine and tyrosine. 



(6) Mono-amino-Acid with Indole Nucleus : Trypto- 

 phan. ^J^, 



(7) Hexone Bases or Diamino-Acids : Lysiril, 

 arginine, and histidine. 



(8) Thio-atnino-Acid : Cystine. 



The chemical analysis of the proteins shows that 

 the various proteins yield different amounts of the 

 amino-acids. Some of the data are shown irr 

 Table I. The peculiarities of each protein are 

 indicated by the figures in heavy type. 



Table I. 



.5 J - 





.si .^S I -51 1 -3 5 



Glycine 



Alanine 



Valine 



Leucine 



Phenylalanine... 



Tyrosine 



Serine 



Cystine 



Proline 



Hydroxyproline 

 Aspartic acid ... 

 Glutamic acid... 

 Tryptophan ... 

 Arginine ... 



Lysine 



riisli(Jine 



Ammonia • ... 



2"I 



37 



0-8 



117 



3-2: 



2-2 



5-8| 



4-5' 



+ I 



7-6 

 1-8 

 I'l 



o 

 1-5 25 

 7-2 0-9 

 9-4 >9-4 

 3-2 2-4 



4-5 0-9 



0-5 



67! 4-0 

 o'3 



1 4 I "O 



i5"6 toT 



3-8 3-2 



60 9-2 



2"5 2-1 



1-6: 13 



19 3 



3-0 



6-8 

 i*o 

 



0-4 

 

 104 

 6-4 

 12 

 1-8 

 



9-3 

 5'o 

 04 

 0-4, 



o 



2-0 



3 '4 

 6-6 



2-4 



I -2 

 0"2 



0-5 

 13-2 



0-6 

 43 7 



I 'O 



32 

 02 

 0-6 

 5-2 



09 



47 

 02 

 60 



2"0 



4-3 

 07 

 0'02 

 4'2 



09 



234 



+ 



47 

 I "9 

 1-8 

 40 



Total ...67-5 66-5 57-o6S'4 83-0 5972 85-4457 81-9 83-1 



o 



9-8 

 1-9 

 19 6 

 6-6 

 3-6 

 i-o 



9-0 



17 



26 2 







1-6 

 o 



0-8 

 3-6 



0-3 



6-2 

 3-8 



5-0 



07 

 127 



+ 



7-1 

 3-0 

 3-0 



2"I 



U 



3-8 

 3-6| 



209' 



3-i| 



2'I 

 03 

 03 



41 

 20 



4-5 

 187 



+ I 



14-458-2 



17 I2*0 

 2*4 I2"9 



In general, the albumin group of proteins contains 

 all the amino-acids, except glycine, in various pro- 

 portions. The globulin group is similar, but contains 

 glycine, and has. In addition, a higher amount of 

 glutamic acid, especially those globulins of vege- 

 table origin. The phospho-proteins resemble the 

 albumins, with no striking preponderance of any 

 single amino-acid. The gliadin group of cereal pro- 

 teins is peculiar in its high content of glutamic acid 

 and proline. The members of the sclero-protein group 

 (horn, hair, and gelatin) are heterogeneous, and- here 

 we may note that silk-fibroin Is composed mainly of 

 three mono-amlno-acids, and Is the very antithesis of 

 sturin (the protein of fish sperm), which is made up 

 of the three hexone bases with no, or very little, 



