BARBARA W. LOW AND JOHN T. EDSALL 



doned by its proponents, because of the finding that two out of 

 the four prohne residues are in a single segment, 15 residues long, 

 at the C-terminal end of the chain (C. H. W. Hirs, W. H. Stein 

 and S. Moore, personal communication). To maintain the 

 structure of a highly compact molecule, such as ribonuclease is 

 known to be, there must be loops spaced at more or less regular 

 intervals along the entire chain. Evidence from ultraviolet 

 absorption near 290 mju indicates that three of the six tyrosine 

 hydroxyl groups in ribonuclease are involved in strong hydrogen 

 bonds (100a, 102a) similar to those observed in ovalbumin (28), 

 and these may play a part in maintaining the native molecular 

 configuration. However, no model yet proposed has succeeded 

 in presenting a unified picture of all the very extensive knowledge 

 now available concerning the structure of ribonuclease. Further 

 research is rapidly extending this knowledge, and there is 

 reasonable hope that within a relatively short time an accurate 

 and fairly detailed picture of the structure will emerge. 



Serum Albumins and Ovalbumin 



Human and bovine serum albumin are very similar in 

 molecular size and physical properties generally (37,38) ; the 

 general pattern of amino acid composition for the two is highly 

 similar, although they difi'er quite definitely with respect to 

 certain amino acids (19). The major portion of both albumin 

 preparations consists of mercaptalbumin (50) (written as ASH 

 for brevity) with a single free sulfhydryl group ; mercaptalbumin 

 is readily separated from the rest of the albumin fraction by 

 crystallization as the mercury derivative, AS — Hg — SA. The 

 molecular weight has been variously estimated, but certainly lies 

 within a rather narrow range, 65,500 to 69,000 (37,38,64,97). 

 On the basis of the lower figure there are approximately 550 

 amino acid residues in either the human or bovine molecule. 

 The most careful determinations of A^-terminal groups have 

 revealed only one per molecule. This is aspartyl in both 

 proteins (104,105), although the next adjoining residue is alanyl 

 in human, threonyl in bovine serum albumin. There are 17 



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