PYRIDOXINE 



of the ingested pjnddoxine and must therefore be taken into account 

 in studies on the metabolism of pyridoxine. 



Scudi et al.,'^ for example, found that man and the dog excreted 

 a conjugated form of pyridoxine, possibly a glycuronate or ethereal 

 sulphate formed by attachment of the conjugating group to the 

 3-hydroxyl group ; this substance was not present in the urine of the 

 rat. A second conjugated compound, apparently derived from 

 4-pyridoxic acid was isolated from the urine of humans and dogs. 

 4-Pyridoxic acid (2-methyl-3-hydroxy-5-hydroxymethylpyridine-4-car- 

 boxylic acid, page 344) was identified as a constituent of human 

 urine by J. W. Huff and W. A. Perlzweig.^ 



These new facts were taken into consideration by Johnson et al.^ in 

 studying the effect of tropical conditions on the excretion of pyri- 

 doxine. Young men were maintained for eight hours a day at a 

 temperature of 37-8° C. and a relative humidity of 70 %. Of the 

 vitamin Bg excreted, 85 % was in the form of 4-pyridoxic acid, 4-0 

 to 4-5 % was pyridoxine and 7 to 8 % was " pseudo-pyridoxine ". 

 The amount excreted in the sweat was one-fifth of that in the urine, 

 and the proportion of the different forms was approximately the 

 same. When the diet was supplemented by 8 mg. of pyridoxine per 

 day, 50 % of the supplement was recovered unchanged and 50 % as 

 the metabolite. The amount excreted in the urine was eight times 

 that excreted in the sweat. 



The metabolism of the three forms of vitamin Bg in humans was 

 studied by J. C. Rabinowitz and E. E. Snell.^" Pyridoxal, pyridox- 

 amine and pyridoxine were estimated in the urine microbiologically 

 (page 313), and pyridoxic acid fiuorimetrically.® The predominant 

 metabolite when any of the three substances was fed was pyridoxic 

 acid. Next in amount were the unchanged substances when either 

 pyridoxine or pyridoxal were fed, but pyridoxamine yielded pyridoxal 

 and pyridoxamine in approximately equal amounts. The highest 

 recovery with a 70 to 80 mg. test-dose was 70 % with pyridoxal ; 

 pyridoxine gave a 45 % and pyridoxamine a 31 % recovery. Normal 

 urine contains no pyridoxine and variable amounts of the other 

 compounds, at least 90 % being pyridoxic acid. 



Vitamin Bg-deficient rats excreted a substance that gave a green 

 pigment with iron ; and the excretion of this substance ceased within 

 a few hours after administering pyridoxine.^^ Vitamin Bg-deficient 

 dogs excreted a similar chromogen.^^ The nature of these excretion 

 products is discussed further on pages 330, 336. 



No attempt appears to have been made to estimate vitamin Bg 

 levels in human blood, but assays with 5. carlsbergensis have been 

 carried out on the blood of monkeys. ^^ After two weeks on a vitamin 

 Bg-deficient diet, the blood level fell to 2 to 3 /xg. per 100 ml. Controls 



326 



