T H E Catabolism of Homologous and Heterologous 7s Gamma Globulin Fragments* by Hans

Digestion of 7S antibodies with papain and pepsin results in the formation of fragments which retain biological activity. I t has been demonstrated by Porter that papain in the presence of cysteine splits the molecule into three fragments of about equal size with a sedimentation coefficient of 3.5S and a molecular weight of approximately 50,000 (1). Digestion of the 7S gamma globulin with pepsin has been shown by Nisonoff et al. to result in an active antibody fragment of a sedimentation coefficient of 5S and a molecular weight of 106,000 (2). The remaining part of the molecule is digested by pepsin into dialyzable polypeptides. In the last few years a number of investigators have been able to localize certain biological features of the 7S gamma globulin in these subunits obtained by enzymatic digestion. Papain fragments I and II, also called the S (slow) fragments because of their slow electrophoretie mobility, each carry an antibody combining site (1). The pepsin fragment is composed of two antibody-combining sites linked by a disulfide bond which can be easily reduced to form two smaller fragments closely resembling the papain fragments I and I I (2). The papain fragment III , or the F fragment (fast), does not interact with the antigen but is responsible for other biological functions of the antibody, such as complement fixation (3, 4), transmission to the fetal circulation (5), and fixation to the skin (6).