IV 



VITAMIN BIOSYNTHESIS 



I II 



rather than for the synthesis of squalene from acetate. The need for the soluble 

 fraction in cholesterol synthesis is probably attributable to the requirement for 

 glycolytic enzymes and acetate activating enzymes. The microsomes, which 

 contain over 80% of the total cytoplasmic cholesterol, also contain most of the 

 newly synthesized cholesterol. Over 90% of the newly formed liver cholesterol 

 obtained at short time intervals after the administration of labelled acetate to rats 

 is also in the microsomal fraction (Bucher and McGarrahan, 1956). When er- 

 gosterol or Ay-dehydrocholesterol is exposed to ultraviolet light, a complex series 

 of reactions takes place involving the rupture of the B ring and leading to the 

 formation of vitamin D2 and vitamin D3, respectively. 



2. Ascorbic acid 



The synthesis of ascorbic acid is markedly accelerated in rats treated with chlore- 

 tone. The administration of labelled D-glucose to such rats is followed by the ex- 

 cretion of labelled L-ascorbate in the urine. When D-glucose- i-^'^C is the substrate, 

 the ascorbate is labelled on carbon 6, whereas with D-glucose-6-^''C, the l- 

 ascorbate is labelled on carbon one. Thus, carbon 6 enters the ascorbate molecule 

 by the same molecular pathway as that followed by carbon i. The conversion 

 is apparently direct, with the maintenance of the steric configuration around 

 carbons 2 and 3 of glucose (carbons 5 and 4, respectively, of ascorbate; Horowitz 

 and King, 1953b). D-glucose- 1-^^*0 is also converted to ascorbate-6-^''C in the 

 normal rat (Burns and Mosbach, 1956). 



On the other hand, there is evidence against the possibility that L-sorbose is a direct 

 L-ascorbate precursor (Isherwood et al., 1953). L-sorbose-6-'*C is converted to ascorbic 

 acid labelled equally on carbons i and 6 and appreciable label is observed in carbons 2 and 

 5 of ascorbate. Thus, the conversion of L-sorbose to ascorbic acid follows the fragmentation 

 of the L-sorbose to three carbon compounds (Burns et al., 1955). 



D-glucuronic acid and L-gulonic acid derivatives may be intermediates in the 

 formation of ascorbate from glucose. Uniformly labelled D-glucuronolactone-^'*G 

 is more efficiently converted to ascorbate in the chloretone treated rat than is 

 D-glucose itself (Horowitz and King, 1953a). D-galacturonic acid or L-galactono- 

 y-lactone derivatives may also be ascorbate precursors (Fig. 50; Isherwood et al.. 



D-Glucurono- 

 y - lactone 



7" -lactone 



Fig. 50. Postulated precursors of ascorbate. 



D-Galacturonic 

 acid - j^-lactone 



Literature p. 124 



