522 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



become translated into production of different protein patterns has hardly been 

 formulated as a tangible problem. Changes of the state of proteins by metabolic 

 reactions are discussed on p. 536 and it may be that similar alterations of the PFS 

 would lead to corresponding changes in the proteins produced. 



(b) Apparent quantitative aspects of protein formation in embryonic cells 



In discvissing the quantitative aspects of protein formation in embryos, factors 

 have to be considered which can usually be disregarded in similar work with 

 fully differentiated tissues. As was pointed out in the preceding section there is 

 little evidence to support or to reject the assumption that proteins having appar- 

 ently similar functions at the beginning and at the end of a developmental period 

 consist actually of identical types of molecules. Certainly in the case of hemo- 

 globin it has been recognized for some time that the fetal protein differs in many 

 respects from the protein in the mature erythrocyte (Haurowitz and Hardin, 1954). 

 Obviously, similar differences could occur in the case of any enzyme or non- 

 enzymatic protein. For example, the substrate concentration curves for alkaline 

 phosphatase of chick muscle may indicate some change during the transition from 

 the embryonic to the more differentiated state of the tissue (Konigsberg and 

 Herrmann, 1955). Generally speaking, for proteins with enzymatic activity, a first 

 approach towards demonstration of such changes could be made by measurements 

 of the Michaelis-Menten constant of the enzyme preparation. With non-enzymatic 

 proteins the identity might be established immunologically by obtaining quantita- 

 tive precipitation curves. The establishment of the identity of proteins at different 

 stages of development with either enzymatic and quantitative immunological 

 methods must, in any event, serve as a necessary prerequisite for subsequent 

 quantitative analysis. 



Whatever methods are used (enzymatic, immunological, etc.), the measurements 

 of the changes in the actual quantities of protein are not necessarily indices of the 

 protein forming capacity of the respective cells. The accumulation of proteins 

 measured in this way is merely the balance of synthesis and breakdown of the 

 investigated protein. Changes in the accumulation rate of a protein can be due to 

 any one of the several possible combinations in the magnitudes of these two 

 processes. An increased rate of accumulation can be due to increased synthesis, 

 decreased breakdown or both and conversely a decreased accumulation can 

 be brought about by decreased synthesis, increased breakdown or both. Fac- 

 tors controlling the accumulation rates could thus act on either of these two proc- 

 esses. An approximate estimate of the relationships of synthesis and breakdown 

 to the accumulation of proteins can be obtained by an analysis of the incorporation 

 of labelled amino acids into proteins. In order to obtain interpretable results certain 

 methodological prereqviisites must be observed, (i) Since the state of proliferation 

 and differentiation is different in different organs of an embryo, data obtained on 

 whole embryos are of limited value. (2) The incorporation measurements should 

 be carried out on individual isolated proteins. During differentiation the rates 

 at which different proteins accumulate change markedly and incorporation data 

 on a protein mixture which shows no apparent change may be the result of an 

 increased rate of incorporation in one protein and a decreased rate in another. 



