364 THE BIOCHEMISTRY OF B VITAMINS 



tainty that a similar mechanism is involved normally in PABA metab- 

 olism. 



Regarding vitamin B 6 , apparently small amounts of all three forms 

 may be excreted in the urine. The major metabolic product of pyridoxine, 

 however, is 4-pyridoxic acid, which accounts for about five-sixths of the 

 known vitamin B 6 metabolites in the urine. 



COOH 

 I 

 HO-^\-CH 2 OH 



CH : 



o 



4-Pyridoxic acid 



Urinary choline appears to be entirely in the free state, although there 

 does not seem as yet to be ample evidence to substantiate the absence 

 of other derivatives. Practically nothing is known of the breakdown 

 products of biotin, or "vitamin Bi 2 ," although the latter is said to be 

 stored by both normal and pernicious anemia patients, even when it is 

 administered in high dosage. 187 Whereas a variety of biotin metabolic 

 products have been suggested, the recent demonstration of the biotin- 

 like activity for microorganisms of a number of structurally unrelated 

 compounds (p. 173) seems to cast serious doubts on the existing data 

 concerning the presence of biotin degradation products in urine. 



Thus at present there are many gaps in our knowledge of the break- 

 down products of the B vitamins. These gaps may reasonably be ex- 

 pected to be filled in the years immediately ahead, largely by the use of 

 isotopic tracer techniques, which have already been so preeminently suc- 

 cessful in problems of this kind, and also by means of the rapidly devel- 

 oping technique of paper chromatography, which is so well adapted to the 

 separation and identification of small amounts of structurally similar 

 compounds. As in many other cases discussed in this monograph, this 

 field is one in which true progress has barely commenced. 



Excretion 



A great many of the aspects of B vitamin excretion have been discussed 

 previously in regard to the assessment of B vitamin requirements (p. 

 254) , the effects of climate and other factors upon the requirements 

 (p. 269), the concentration in milk (p. 347) which may be regarded as 

 an excretory product, and with regard to the breakdown products of the 

 B vitamins (p. 361) . Still other aspects are more appropriately discussed 

 in the consideration of deciency states. Consequently our present discus- 



