CELL LIPIDS 45 



any striking differences between these and higher organisms when all 

 proteins are hydrolysed together. 



The biological utilization of fungal proteins has been studied in two 

 contexts. Mycelium added to the soil is decomposed by the microflora 

 about as rapidly as are other organic materials (78). Norman (115) 

 could find no evidence of a decay-resistant nitrogenous fraction in my- 

 celium (89). In animal nutrition, the utilizability of fungus proteins 

 is generally low, i.e., they do not support growth of the rat (49-52, 

 162, 163, 189). The nutritional value of fungal proteins is reviewed 

 by Thatcher (175); the mycelium of some fungi is toxic to laboratory 

 animals, but the underlying mechanism — whether an amino acid de- 

 ficiency or a toxic substance — is not known. 



The non-protein nitrogen of the fungus cell consists of chitin, al- 

 ready discussed, nucleic acids, free amino acids, and miscellaneous 

 nitrogen compounds of low molecular weight. The nucleic acid of 

 Penicillium spp. accounts for 4 per cent of the total nitrogen and 

 about 1 per cent of the dry weight (1, 165). These findings cast some 

 doubt on earlier claims, reviewed by Akasi (1), of high nucleic acid 

 concentrations in fungi. Spores of Penicillium spp. contain 158-592 

 fig ribonucleic acid per billion (33). The conidia of Aspergillus nidu- 

 lans contain measurable deoxyribonucleic acid and the amount is, as 

 in other organisms, twice as high in diploid as in haploid cells (77). 



Free amino acids occur in fungus mycelium (31, 68, 109, 120, 127, 

 158, 190) and are also found in the culture medium (32, 109, 135). 



The other nitrogenous constituents known in fungi include many 

 different and unrelated compounds, several of which are discussed in 

 Chapter 8. Nitrogen bases and amines have been found (90, 171, 

 197) possibly arising by degradation of larger molecules during analy- 

 sis. Cyclic choline sulfate has been isolated from fungi (31, 167, 192). 

 Urea appears to be a frequent constituent of the higher basidiomycetes 

 (88), and constitutes 3.9 per cent of the total nitrogen of Aspergillus 

 fischeri mycelium (67). Allantoic acid has been reported in appreci- 

 able amounts in the sporophores of Agaricales (54) and may be one 

 precursor of urea (Chapter 8). 



6. CELL LIPIDS 



The total lipid content of fungus mycelium, usually expressed as 

 a percentage of the dry weight, is so strongly affected by cultural con- 

 ditions and age that any set of figures is misleading. The effect of 

 these conditions is brought out especially well in a survey by Walker 



