17 



The isolation of an immunologically active 

 fragment of bovine serum albumin 



R. R. PORTER 



National Institute for Medical Research, Mill Hill, London, N.W.7 



It has been recognized for about thirty years that proteins may possess 

 intrinsic biological activity as enzymes, hormones, antigens and antibodies, 

 and Northrop (1932) was perhaps the first to formulate and try to answer 

 the next question : Is the whole protein molecule essential for these activities 

 or is it a property of only part of the molecule? Northrop attempted to 

 hydrolyse crystalline pepsin without loss of activity but failed. Success was 

 soon achieved with antibodies (horse anti-diphtheria toxin serum) by 

 Parventjev (1936), Petermann and Pappenheimer (1941), and Northrop (1942), 

 the last two isolating approximately half molecules which would still com- 

 bine with the antigen. Later a fifth part of rabbit anti ovalbumin antibody 

 was obtained, by papain digestion, which retained the specific combining 

 power (Porter, 1950). 



Recently the partial degradation of several enzymes without loss of 

 activity has been reported, e.g. pepsin (Perlman, 1954), papain (Hill and 

 Smith, 1956) and ribonuclease (Richards, 1955; Kalnitsky and Rogers, 1956; 

 Rogers and Kalnitsky, 1957). The degradation, in several cases, appeared 

 to be very considerable, although the isolation of the fragment and proof 

 that the active fragments were as small as other evidence appeared to suggest 

 has not yet been described. Some terminal amino acids may be removed 

 from the hormones insulin (Harris and Li, 1952), adrenocorticotrophin and 

 growth hormone (Harris, Li, Condliffe and Pon, 1954) and also from tobacco 

 mosaic virus (Harris and Knight, 1952) without loss of activity. 



The combining centre of a protein antigen might appear to offer the 

 easiest opportunity of isolating a specifically active fragment of protein, 

 as Landsteiner's work with artificially conjugated antigens showed that the 

 specificity is directed, in these compounds, to the small attached groups — 

 the haptenes. This he showed by using the haptene to specifically inhibit 

 the combination of the conjugated antigen with its antiserum. However, a 

 very considerable excess of haptene is required to achieve inhibition (a molar 



