36 THE BIOCHEMISTRY OF B VITAMINS 



B 6 " and pyridoxine are by no means synonymous, and that pyridoxamine 

 is fundamentally just as important as pyridoxine, and pyridoxal is even 

 more so. 



The question of the combined forms of "vitamin B G " was not answerable 

 until these findings were made, and all conclusions based upon the sup- 

 position that vitamin B 6 and pyridoxine are one and the same were made 

 obsolete by these discoveries. 



Pyridoxal phosphate, presumably pyridoxal esterified with phosphoric 

 acid in the 5-position, is now known to be an important coenzyme in- 

 volved in decarboxylation of amino acids 69 and in transamination. 70 - 71 - 72 

 This constitutes one of the most important combined forms of vitamin 

 B c and is itself the prosthetic group of a number of enzymes, some of 

 which have been purified. 71 Pyridoxal readily forms combinations with 

 amino acids in vitro, and such combinations may be important in nature. 73 



Pyridoxamine phosphate is another important bound form of vitamin 

 B G , and appears to be the principal form occurring in yeast, where it is at 

 least partly in the free state. 74 It is not hydrolyzed by alkaline extraction 

 and may be extracted by this means from liver and grass preparations. 

 In these latter materials it exists in combined forms presumably with 

 proteins, and appears to constitute the predominant form of the vitamin. 

 Pyridoxamine phosphate is also reported to be present in certain trans- 

 aminase preparations. 75 



Pyridoxine, rather than pyridoxal or pyridoxamine, appears to be the 

 principal form of the vitamin in certain seeds (rice and wheat) , but little 

 is known regarding its mode of combination. 70 These cereals, it will be 

 noted, are not metabolically active materials, as are those containing 

 predominantly pyridoxal and pyridoxamine. 



The complete extraction of all forms of vitamin B c in the free con- 

 dition from tissues in general must take into account each of the forms. 

 Pyridoxamine phosphate is stable toward alkaline hydrolysis and is 

 hydrolyzed less readily than pyridoxal phosphate in acid medium. 74 

 Stronger acid {2N) is far less effective in the release of free vitamin from 

 yeast than is acid of lower concentration (0.055A0 . 77, 7S Rabinowitz and 

 Snell 78 found that for many materials autoclaving for 5 hours in 0.055iV 

 HC1 at 20 lbs. pressure gave maximum yields. A rice bran concentrate, 

 however, required hydrolysis with 2N acid, and dried green peas and oats 

 yielded less vitamin B 6 with the regular procedure than animal tests 

 indicated to be present. This discrepancy may be due to intestinal 

 synthesis. 



The problem of extraction of all forms of vitamin B e in the free form 

 is made less acute by the fact that there appears to be no destruction by 



