FREE AMINO ACIDS IN INSECTS I31 
aegypti females was increased by isoleucine, but not by a combination of methionine, 
valine, phenylalanine, tryptophane, threonine, leucine, arginine, histidine and lysine. 
Dimonp eé¢ al.4®, who also worked on this mosquito species, reported that for egg-laying 
the following amino acids had to be included in the diet: arginine, isoleucine, leucine, 
lysine, phenylalanine, threonine, tryprophane and valine. There was a reduction 
in the number of eggs laid if histidine, cystine or methionine was eliminated from the 
diet. These results suggest that yolk proteins are synthesized directly from amino 
acids. From their study on the house fly, ASCHER AND LEviNSoN?* concluded that 
protein reserve in the larvae could not be utilized by female adults for egg production. 
The presence of y-amino--butyric acid has been detected in a number of in- 
sects}, 39, 41, 20, 64. According to KATinG? this unusual amino acid participates in 
N-assimilation, especially in the transamination of aminocarbonic acid. Recently, 
McLENNAN! and VAN DER KLOOT AND Rossins?! reported from their studies on the 
muscle contraction in crayfish, that y-aminobutyric acid works as transmitter sub- 
stance of inhibitory neurons. It remains to be investigated whether it has a similar 
function in insects. 
In a recent paper SLOPER!” identified the presence of a substance rich in protein- 
bound cystine or cysteine in the neurosecretory system of Leucophaea maderae. This 
finding is of particular interest because, as already mentioned, these amino acids, in 
addition to their close intermediary relation to methionine, serve as important sources 
of sulfhydryl groups for the synthesis of coenzymes and hormones (see p. 122). 
In their studies on the iodine metabolism in insects, LIMPEL AND CASIDA?26, 127 
found that if radioiodide was injected into the cockroach Periplaneta americana, 
most of the iodine was excreted as the iodoamino acid monoiodohistidine. When 
radioactive iodine was given in the form of monoiodohistidine, di-iodohistidine could 
be detected in the excreta. These experiments strongly suggest that histidine serves 
as a detoxicating agent in this insect. 
Glutamine is an important component in the hemolymph of insects. It is formed 
from ammonia and glutamate in the presence of ATP. From a number of experiments 
on various animals, it is known that this substance participates in transamination!*!, 188 
and transpeptidation®: 154, But it can also be incorporated directly into proteins!® 167, 
In both Bombyx 4° and Schistocerca?, glutamine has been shown to be an effective 
amino donor in transamination reactions (see below). Furthermore, it is involved in 
the synthesis of uric acid and glucosamine”, 12°, The latter substance participates 
in the formation of chitin, and was also found in the protein hydrolyzate of the salivary 
gland secretion of Drosophila™*. In mammals, evidence also has been presented to 
show that glutamine serves as a detoxicating agent for a number of aromatic acids 
and ammonia!*4, 182, Whether it fulfills the same function in insects, is not clear. 
The metabolism of amino acids in insects is a complex problem. As in other animals, 
transamination is a common phenomenon in insect tissues!’. It involves mainly the 
formation of glutamate from a-ketoglutarate with aspartate and alanine as amino 
donors. Transaminase activity has been demonstrated in the silkworm Bombyx 
mort}, 23, 76. More recently, KILBY AND NEVILLE1: 112 presented evidence for the 
occurrence of transaminase in fat body, gut and Malpighian tubules of the locust 
Schistocerca gregaria. These enzymes, similar tothose in mammals, effect transamination 
of a-keto and a-amino acids. According to these authors the transaminases were 
present in the mitochondrial and soluble fractions of tissue preparations. Furthermore, 
References p. 132/135 
