CHEMICAL PATHWAYS 115 



insulin and into ribonuclease in slices of beef pancreas. This time, each 

 protein was split into many peptides ; the specific radioactivity of the same 

 amino acid was notably different in the different peptides originating from 

 the same protein (Vaughan and Anfinsen, 1954). More recently, non- 

 uniform labelling was reported for collagen (Gehrman et al., 1956) and 

 procollagens (Orekhovich et al., 1959), for silk fibroin (Shimura et ah, 1956), 

 for amylase in B. siibtilis (Yoshida and Tobita, 1960), for haemoglobin in 

 the rabbit (Kruh et al., 1960) and for cytochrome-c in isolated muscle 

 mitochondria (Simpson, personal communication). In other cases, how- 

 ever, no inequalities of labelling were detected (Muir et al., 1952; Heim- 

 berg and Velick, 1954; Askonas et al., 1955; Simpson, 1955) but the 

 experiments were made with intact animals and the incorporation was of 

 relatively long duration. Non-uniform labelling is best observed for short 

 periods of amino acid uptake and with poorly active or damaged systems. 



The meaning of unequal labelling is not clear. Good discussions on the 

 subject have been presented by Steinberg et al. (1956) and by Borsook 

 (1956). The first interpretation proposed was that amino acids are first 

 incorporated into various oligopeptides or peptide derivatives which are 

 later combined to make a protein. According to the size of each peptide 

 pool and to their turnover rate, a given labelled amino acid would be 

 diluted to difi^erent degrees in each oligopeptide, and the protein formed by 

 association of the peptides would not be uniformly labelled. The incorpora- 

 tion is so low in most of the in vitro systems considered that a pool of 

 peptides amounting to some 2 per cent of the free amino acid pool could 

 account for the experimental data. 



Another way of explaining non-uniform labelling is to assume that in the 

 presence of protein forming system, the peptide linkages of proteins can 

 be labilized in such a way that an amino acid comprised within the poly- 

 peptide chain can exchange with free amino acids (Chantrenne, 1952; Gale, 

 1953; Borsook, 1956). Differences in exchange rate at different positions 

 in the protein chains would explain differences in specific radioactivity. An 

 alternative interpretation, which avoids the assumption of exchange 

 processes for which there is no compelling evidence, is that the time 

 required for making a protein is long in the poorly active system considered ; 

 in this sense that each individual polypeptide molecule stays on the weaving 

 machine long enough for the specific radioactivity of an amino acid, e.g. 

 glycine, to change considerably between two additions of this amino acid 

 at different places along the polypeptide (Dalgliesh, 1953). If protein 

 synthesis takes places by such a 'stepwise template process' (Steinberg et ah, 

 1956) one should expect to find peptide intermediates bound to some 

 structures in the experimental systems in which unequal labelling is 

 observed. 



In the last few years, the occurrence of nucleotide-amino acid and 



