INOSITOL 339 



deficient mutants and from the accumulation of intermediates by de- 

 ficient strains. Thus, one nicotinicless mutant cannot use quinolinic 

 acid and accumulates detectable amounts of this compound, indicating 

 that it is blocked in the conversion of quinolinate to the vitamin (19, 

 102). Other mutants accumulate 3-hydroxyanthranilic acid (18) or a 

 derivative of kynurenine (328), or convert one intermediate into others 

 (330). 



As indicated in Figure 4, the steps from 3-hydroxyanthranilic acid to 

 nicotinic acid are still uncertain and may follow one of the two pos- 

 sibilities shown, via unstable intermediates (88). 



Preliminary studies of nicotinicless mutants of Aspergillus nidulans 

 (195) suggest that a second and shorter pathway occurs in addition to 

 the reactions of Figure 4; this shorter sequence cannot be detected in 

 Neurospora crassa (187). 



The Neurospora pathway is probably followed also in the rat, for 

 which some direct enzymatic evidence is available (257). In the bac- 

 teria so far studied, the pathway does not appear to be applicable (328). 

 Evidence on other fungi is limited to the finding, mentioned earlier, 

 that tryptophan and certain other intermediates of the Neurospora 

 pathway replace nicotinic acid for Trichophyton equinum (51, 79). 



10. INOSITOL 



Requirements for inositol (meso-inositol, /-inositol, m/yo-inositol) are 

 rather rare in fungi; complete or partial deficiencies have been reported 

 in, for example, yeasts (30), Trichophyton faviforme (82), Lophoder- 

 mium pinastri (123), Sclerotinia camelliae (8), Rhizopus suinus (238), 

 Eremothecium ashbyii (243, 331), Diplocarpon rosae (253), and Dia- 

 porthe phascolorum var. bataticola (283). Inositolless mutants have 

 been found in Neurospora crassa (9) and other fungi (17, 70, 121). 



It is doubtful whether inositol should be considered a vitamin as the 

 word is used here. No coenzyme function is known in any organism; 

 rather, the finding that most of the inositol of Neurospora crassa myce- 

 lium is associated with phospholipid suggests that it is a structural 

 component of the cell (74). 



The magnitude of the inositol requirement is much greater than that 

 of the other vitamins (136, 259, 281), again suggesting a structural 

 rather than a catalytic role. The weight of dry cells per gram thiamine 

 needed is about 0.5-2.5 x 10 6 , and the comparable value for inositol is 

 only 600-900 (65). Figure 5 illustrates the growth response to inositol. 



The inositol requirement is highly specific (125, 242). Phytin, the 

 salt of inositol hexaphosphoric acid, replaces inositol for Eremothecium 



