48 MACROMOLECULAR COMPLEXES 



iodine, tyrosine is the only amino acid in collagen capable of iodina- 

 tion. A further pointer in this direction was the report by Grass- 

 mann et al. (1956) of a tyrosine-containing peptide, the Sf peptide, 

 in tryptic digests of thermally denatured collagen.'' 



In view of the above considerations, it seemed desirable to follow 

 the distribution of tyrosine in paper-curtain electrophoresis of tryp- 

 sin-treated collagen by iodination of the digest with iodine contain- 

 ing the radioactive isotope P'^V Figure 26 shows the distribution of 

 radioactivity observed in electropherograms of a control run with 

 trypsin alone, a run with trypsin and native ( undenatured ) collagen 



P/^c 



Fig. 28. Two-dimensional paper chromatogram of an acid hydrolysate of 

 Fraction I from Run B-1 shown in Figs. 26, 27, run in phenol-o-cresol (1:1) 

 and n-butanol-acetic acid-H^O. Note the relatively large amounts of tyrosine 

 (Tyr) and mono-iodo-tyrosine (MIT). (From Hodge ef o/., 1960.) 



* Recent results of Grassmann (personal communication) indicate that the Sf pep- 

 tide arises as a result of chymotrypsin activity (an impurity in the trypsin used in 

 previous experiments ) rather than from the activity of trypsin itself. 



