II UTILIZATION OF PROTEIN PRECURSORS 503 



observed in explanted embryos. Even concentrations of 2.0 mg/ml of the medium 

 caused only a slight retardation of growth. Thienylalanine, the analog of phenyl- 

 alanine, however, gave rise to distinct anomalies. The main deviations observed 

 were: a malformed brain, open and frequently distorted neural tubes, and small, 

 pale staining somites which showed cavitation in the sclerotomes. These abnor- 

 mal features of the early developmental period were accompanied by a marked 

 retardation of overall growth. The latter effect could be prevented, at least in 

 part, by the addition of the normal amino acid, phenylalanine, but the formation 

 of abnormalities was irreversible. 



The analogs of valine and leucine caused very little growth retardation. Their 

 interference with development was restricted almost entirely to a disturbance of 

 somite development. Somites appearing about two to three hours after explan- 

 tation were no longer separated from each other but retained the structure of an 

 uninterrupted plate. The justification for referring to this unsegmented plate as 

 an abnormal somite development is derived from the observation that at least 

 with some of the leucine- valine analogs formation of the dermomyotome and of a 

 sclerotome could be observed. Thus, the abnormal plate formed is not completely 

 devoid of somite characteristics. The effect of the leucine analogs could be reversed 

 not only by leucine and valine but, to a greater or lesser extent, by other amino 

 acids which contain a chain of more than three carbon atoms. Serine and alanine 

 were found to be much less effective in alleviating the abnormality. 



In view of the absence of any effect by ethionine, it seemed desirable to test this 

 analog in systems where the mechanism of its effect on protein synthesis could 

 be explored more readily. In using the older chick embryo, it was found that fol- 

 lowing an injection of ethionine into the yolk or onto the chorioallantoic mem- 

 brane, growth, measured in terms of body weight, was not retarded until after the 

 7th day of development. These findings were corroborated by the experiments 

 carried out independently by Karnofsky, et al. (1955). After the 7th day of chick 

 development, the increase of bodyweight was markedly reduced and at later 

 stages certain pathological symptoms developed which are of less importance in 

 the present context. 



These results seemed to present some rather perplexing contradictions. If it is 

 assumed that the effects of amino acid analogs are due to interference with protein 

 formation, one would have expected that any one of the tested analogs would 

 inhibit synthesis of all proteins with the result of a general growth inhibition but 

 without specific effects in any particular tissue. Instead, the one analog (ethionine) 

 was found to be practically ineffective in explants tested while other analogs 

 exhibited rather specific effects on various organ systems. 



As an explanation of these discrepancies several possibilities can be considered. 

 First, it is possible that various analogs inhibit the rate of formation of different 

 proteins to a different extent depending, possibly, upon the number of molecules 

 of the corresponding normal amino acid in the respective protein. This could 

 lead to a change in the proportion of different proteins in different cell types 

 resulting in the abnormal reactions of various cells in this change. 



In exploring the mechanism of the effect of the analogs leucine and methionine, 

 the incorporation of labelled glycine into proteins of explanted chick embryos 



Literature p. sjg 



