I22 P. S. CHEN 
termediary metabolic pathways and fulfill special functions in insect metabolism. 
The two amides, asparagine and glutamine, are of wide distribution in insects. Both 
of them are able to transfer amino groups in the transamination reactions (see p. 131). 
There is, however, very little information available on the peptides. In those studies, 
where no hydrolysis of samples has been carried out, some peptides have been possibly 
designated simply as unknown substances. 
Other physiological factors may account for the high degree of variations in amino 
acid concentration found in insects. In higher organisms Hatz* observed that injection 
of amino acids such as tryptophane, tyrosine and histidine causes a temporary rise of 
alanine. AUCLATR® also emphasized that in Blatella germanica certain amino acids, 
when fed alone, could affect the quality and concentration of free amino acids in the 
blood, but other amino acids did not show this effect. The concentration of different 
amino acids are obviously interrelated. For mammals L1, GESCHWIND AND Evans!”° 
reported that growth hormone effects a distinct drop in amino acid contents of the 
blood. Although no such experiments have been yet carried out on insects, it 1s con- 
ceivable that the complicated hormone system in these organisms, especially during 
development, influences directly or indirectly the patterns of free amino acids. 
The most obvious effect is of course the nutritional state. The study of CHEN AND 
Haporn” on Drosophila larvae showed that during starvation essential amino acids 
such as valine and leucine disappear completely from the chromatogram, whereas the 
contents of certain peptides increase rapidly. They observed that larvae fed on saccha- 
rose alone had a very high concentration of alanine, which was obviously synthesized 
from pyruvate through transamination. In this connection, one has to consider also 
the absorption of different amino acids contained in the diet. For the German cockroach 
Blatella germanica, AUCLAIR® reported that Dr-homocystine could not be absorbed. 
Working on the locust Schistocerca gregaria, TREHERNE!” reported that among the 
amino acids tested by him, glycine and serine were absorbed most rapidly from the 
mid-gut caeca. The author concluded that in this insect, in contrast to the active 
transfer in mammals, the amino acids enter into blood probably through “a diffusion 
gradient established by net movement of water molecules into the hemolymph”. 
Several S-containing free amino acids like cystine, cysteine, cysteic acid and me- 
thionine have been recorded in insect blood. These substances are interrelated in inter- 
mediary metabolism (see GILMOUR®®), and serve as important sources for the sulfhydryl- 
group in coenzymes and hormones (BARRON!®; GREENBERG®?). Adult female mos- 
quitoes are especially rich in methionine sulfoxide (CHEN*!). According to STEKOL!? 
this compound can replace methionine for growth. For the common house fly Musca 
domestica, a tracer study showed that methionine could not be formed from cystine 
(HitcuEy, Cotry AND HENry®”). Taurine was, however, radioactive in flies fed with 
5S-cystine. It is interesting to notice that in the ciliate Tetrahymena geleiz, in contrast 
to mammals, methionine cannot be formed from homocystine (GENGHOF”). The 
metabolic pathways of these compounds in invertebrates are obviously different from 
those in higher organisms. 
FREE AMINO ACIDS AND TAXONOMY 
Paper partition chromatography has been employed for taxonomic purposes. This 
technique is simple and sensitive for detecting small quantities of biological sub- 
References p. 132/135 
