I 12 



INTERMEDIARY METABOLISM AND GROWTH 



1954a). The feeding of D-glucurono-y-lactone, L-gulono-y-lactone, D-galacturonic 

 methyl ester, and L-galactono-y-lactone to cress seedHngs or the sidministration of 

 these compounds to rats is followed by increased ascorbate synthesis. No other 

 sugar acid y-lactone, of many tested, behaves in this way. 



Ascorbate synthesis differs somewhat in mung bean seeds from cress seedlings and the 

 rat. Mitochondria of mung bean seeds convert L-galactono-y-Iactone and the methyl ester 

 of galacturonic acid to ascorbate but neither L-gulono-y-lactone, D-glucuronic acid-y- 

 lactone nor free galactonic acid were converted to ascorbate. 



The oxidation of glucose to glucuronic acid takes place in the particle free super- 

 natant of liver tissue (Strominger et al., 1954). UDP glucose rather than free glu- 

 cose is the substrate of this oxidation : 



2DPN^ 

 i) UDPG ^ UDP-ghicuronic + 2DPNH + 2H+ 



This suggests that in the rat, ascorbate synthesis may take place as follows : 



ATP 



2) Glucose » glucose-6-P 



L-gulono-y-lactone <■ 



i 

 L-ascorbic 



UTP 



glucose- 1 -P > UDPG 



D-glucuronolactone 



UDP glucuronic 



2 DPN^ 



J. Thiamine 



On the basis of nutritonial experiments with Neurospora and E. coli mutants, it has been 

 inferred that two pathways of thiamine synthesis exist (Harris, 1955). These are shown 

 in Fig. 51. Neurospora mutant (18558) requires thiamine for growth and accumulates 

 "pyrimidine" (P) in the culture medium. Thiazole (T) can substitute for thiamine in 

 this organism, but not in mutants, 9185 and 85902. The latter organisms require thiamine 

 and accumulate both P and T in the culture medium. Thus, one pathway of thiamine 

 synthesis involves the condensation of P with T to yield thiamine. The growth requirement 

 of two other Neurospora mutants (17084 and 56501) can also be met by thiamine or by 

 P plus T. However, these organisms accumulate an unknown compound, P-t in the 

 mycelium. Moreover, P-t can replace thiamine as a nutrient in strains 9185 and 18558. 



18558 



+ t 



P + T- 



9185 

 85902 



P-t 



B. 



17084 

 56501 



CH2-N*=CH 



i=c> 



I I 



CH3 CHfCHjOH 

 Thiamine 



Fig. 5 1 . Proposed pathways of thiamine synthesis. 



