The Position of Various Cleavages of Rabbit Immunoglobulin G

The 5 S fragments of rabbit immunoglobulin G were prepared by digestion with pepsin or with CNBr. Cleavage of these fragments with trypsin led to the localization of the positions of these splits on the heavy chain. The COOH-terminal residue released by carboxypeptidase A from the papsin-produced 5 S fragment was leucine. The pepsin-5 S fragment was reduced and aminoethylated to yield the 3.5 S fragment. Trypsin digestion of this fragment yielded two decapeptides which represent the COOH terminus of the heavy chain part of the 3.5 S fragment. The major one, obtained in 65% yield, had the sequence Ser-Lys-Pro-Thr-AE-Cys-Pro-Pro-Pro-Glu-Leu. The other decapeptide which was obtained in a smaller yield (20%), had the same amino acid composition but a different carbohydrate composition. When the CNBr-5 S fragment was digested with pepsin and then reduced and aminoethylated, it yielded a 3.5 S fragment. Digestion of the latter product with trypsin released the same decapeptides mentioned above. The COOH-terminal sequence of the CNBr-5 S fragment was shown to be Asp-Thr-Leu-homoserine. Thus, the order of the size of the various fragments produced by different cleavages is: CNBr g pepsin g trypsin. The positions of the various cleavages were correlated with the known sequence of the heavy chain.