GROWTH FACTORS 103 



rubra, can synthesise the thiazole but not the pyrimidine 

 part. The two organisms can he grown together in 

 symbiosis, each producing the component needed by 

 the other. Many organisms, of which Escherichia coli, 

 Proteus vulgaris, Aerohacter aerogenes, Alhaligenes 

 faecalis, Bacillus mesentericus and Thiohacillus 

 thio-oxidans are examples, are independent of added 

 aneurin, being capable of synthesising it themselves. 



It is highly probable that the aneurin is active in the 

 decarboxylation of pyruvic acid, since aneurin pyro- 

 phosphate is loiown to be co-carboxvlase (see 

 Chapter XII). 



Biotin. — ^'<^ , Kogl isolated biotin in 1935 



/"\ 

 / \ 



15' 4' I 



CH 3CH 



I ^ I 



CH2 2CH.(CH2)4.COOH 



from egg yolk and from yeast. It occurs in these sources 

 in very minute quantities, 360 tons of yeast or one and a 

 half million eggs being required to yield Ig. of the crys- 

 talline substance. It has m.p. 230 — 231°C. and 



[a]^2== + 92° (in 0-1 N NaOH). It is probably the most 

 active biological substance known at present ; a dilution 

 of one part in 10^*^ is sufficient to promote half the maxi- 

 mum gro^vth of yeast. It has been synthesised and the 

 synthetic material is indistinguishable from the natural 

 substance in biological activity. It is inactivated by 

 treatment with nitrous acid, oxidising agents or by 

 benzoylation or acetylation. The methyl ester, however, 

 can be utilised in place of free biotin, but in some cases 

 less readily. By reduction in presence of the Raney 

 nickel catalyst the sulphur atom is eliminated and 



8 



