258 p. N. CAMPBELL 



albumin precursor then moves to the hpid membranes where there is a 

 further addition of amino acids to complete the albumin molecule. The 

 albumin then moves to the internal medium of the vesicles where it is 

 securely retained until it is passed through the cell membrane. 



I am grateful for the expert assistance of Miss Barbara Kernot in all 

 the experiments described which were carried out in our laboratory. I am 

 also grateful for the encouragement of Professor F. Dickens. The work 

 described was made possible by a grant to the Medical School from the 

 British Empire Cancer Campaign, 



References 



1. Campbell, P. N., Greengard, O. and Kernot, B. A. Biochem.J. 74, 107 (i960). 



2. Lapresle, C, Ann. Inst. Pasteur 89, 654 (1955). 



3. Peters, T., ]r.,J. Histocheni. Cytochem. 7, 224 (1959) 



4. Porter, R. R., Biochem.J. 66, 677 (1957). 



Discussion 



Porter : I was impressed by the rate at which serum albumin was synthesized 

 after the intravenous injection of labelled amino acids and I wondered how much 

 synthesis took place during homogenization and fractionation of the particles of the 

 microsomes. 



Campbell : This seems unlikely for the conditions which exist during the 

 homogenization and fractionation procedure are not favourable for the incorpora- 

 tion of much [^^C]-amino acid. The concentration of amino acid would be very 

 small owing to the addition of medium, the concentration of ATP and other energy- 

 rich compounds would be low, and the temperature is also low. We hope that the 

 finger-print method may prove to be a useful technique. A mere demonstration 

 of radioactivity in a protein after injection should not really be taken as evidence of 

 complete synthesis. We think that by this method it would be possible to demon- 

 strate a synthesis of complete molecules. 



Davis : I wonder whether the fact that you find uniform distribution of radio- 

 active leucine in serum albumin after only 3 min. might not be compatible with the 

 following alternative interpretation. The individual amino acid molecules might 

 line themselves up at random times, with no one site having a preferential position 

 compared with another in relation to time. That is, the first few amino acids might 

 just as well go on at one end as at the other or even in the middle. When the entire 

 set of slots were filled peptide bond formation would be completed and the poly- 

 peptide chain would zip off. Therefore during that 3 min. the radioactive leucine 

 might have found some templates which were almost completed but missing a few 

 leucines here, and other templates missing a few leucines there. All the proteins 

 that did get produced at that time would average out a random distribution of 

 radioactive leucine. 



Campbell: Yes, that is a most interesting possibility. If it were true we should 

 not find any change in the pattern even when the animal was killed at a very short 



