346 THE BIOCHEMISTRY OF B VITAMINS 



Riboflavin is converted by the red blood cells (probably only nucleate 

 erythrocytes) and most other cells of the body into flavin adeninedinucleo- 

 tide, but this reaction cannot be performed by the plasma. 35 - 38 There 

 is apparently a rather constant equilibrium between the coenzyme levels 

 in cells and plasma; and the level in the whole blood remains quite steady 

 under most conditions. Human blood is said to contain the equivalent of 

 about 21.2 ixg per cent, varying somewhat annually in places where there 

 are marked seasonal dietary changes. 39 The blood plasmas of several 

 Brazilian snakes contain levels ranging from 180 to 300 /^g per cent. 40 



.01 .03 .10 .30 1.0 



REQUIREMENT (MG/kg/dAY) 



Figure 15. Relationship of the requirements by various species for thiamine and 

 pantothenic acid to the blood levels of these vitamins. 



Nicotinic acid and nicotinamide enter the blood, as far as is now known, 

 as such, although it is possible that small amounts of cozymase may 

 escape digestion and be directly absorbed. Blood plasma contains both 

 the acid and the amide, and these are converted to cozymase 41 in 

 nucleate blood cells (for the most part white cells, which contain very 

 little of the free vitamin) . In humans, the blood level normally ranges 

 from 400 to 700 fig per cent, of which about one-third is free and the 

 balance is combined as coenzyme. 42-45 The blood level of nicotinic acid 

 is not generally lowered appreciably in pellagra. 46 Horse blood contains 

 about 180 fig per cent free and 140 ^g per cent combined nicotinic acid, 

 while in the cow the corresponding levels are about 268 and 332 fig per 

 cent, respectively. 



Pantothenic acid is found in the blood of man in concentrations of 

 about 30 fig per cent, 47 and in chicks in levels of about 40 fig per cent 

 (50 fig per cent in the plasma), 48 but little is known of its state or dis- 



