288 THE BIOCHEMISTRY OF B VITAMINS 



vides still another niacin derivative which may figure in meeting the 

 nicotinic acid requirements of man. 110 



It has been previously mentioned that cooking may modify the nutri- 

 tional value of a vitamin source. An interesting example of improved 

 nutrition resulting in this manner is in the case of coffee. Raw coffee con- 

 tains a preponderance of trigonellin over nicotinic acid, but in the roast- 

 ing process the trigonellin is largely converted to nicotinic acid, so that a 

 cup of coffee generally contains from 1 to 2 mg of nicotinic acid. 117 



(4) The Vitamins B 6 



The three major forms of vitamin B 6 are pyridoxine, pyridoxal, and 

 pyridoxamine, to which may be added the functional vitamin form, code- 

 carboxylase, or pyridoxal phosphate, and pyridoxamine phosphate, which 

 is now known to be a nutritional requirement for certain lactic acid 

 bacteria. In addition, "bound" forms exist, and there is increasing evi- 

 dence to suggest the possibility of a variety of vitamin B 6 -amino acid 

 complexes (Schiff bases) with biological activity. Finally the ability of 

 some lactic acid bacteria to employ certain amino acids interchangeably 

 with vitamin B G may eventually be found to have broader biological 

 significance. 118 



In vitamin B G -deficient mice, 119 using xanthurenic acid excretion as a 

 criterion of response, Miller and Bowman found that pyridoxine hydro- 

 chloride returned the excretion to normal within four days, while pyridoxal 

 and pyridoxamine required seventeen and twenty-four days respectively. 

 Growth rates on the three analogues and survival times on lower levels 

 similarly indicated the same order of activities. 



For the rat, it has been shown that the three forms are about equally 

 active 120 in promoting growth and in restoring the ability to convert 

 tryptophan to niacin. 120a It has also been reported that for the rat and 

 chick, all three forms and pyridoxal phosphate are equally active when 

 fed by eye dropper or injected intraperitoneally, 121 although when added 

 to the ration, pyridoxine appears somewhat more active. The three forms 

 are also known to be equally active in promoting growth and blood 

 regeneration in vitamin B 6 -deficient dogs. 122 



Although certain studies would suggest the existence of further ana- 

 logues of vitamin B 6 in nature, 123 the work of Rabinowitz and Snell 123 - 124 

 indicates strongly that the B 6 trilogy, their phosphates, and their protein- 

 bound forms account for the vitamin B 6 content of tissues. Little is as yet 

 known of the biological activity of the various products of vitamin B 6 

 metabolism in mammals, although 4-pyridoxic acid is known to be inac- 

 tive for rats, chicks, and all the bacteria so far tested. 



