EVIDENCE THAT THE HINGE REGION PLAYS A ROLE IN MAINTAINING SERUM LEVELS OF THE MURINE IGG1 MOLECULE

The site of the murine IgG1 molecule that regulates catabolism has rec ently been shown to encompass amino acids that are located at the CH2- CH3 domain interface. The CH2 and CH3 domains are connected to each ot her by a relatively flexible ''mini-hinge'' region, and flexibility in this region could clearly affect the orientation of the domains with respect to each other. The internal movement of the CH2 domain depends on the absence/presence of the hinge disulphide. The increased mobili ty of the CH2 domain relative to the CH3 domain in a hingeless IgG or Fc fragment may result in a conformational change at the CH2-CH3 domai n interface and alter the accessibility of the residues that are invol ved in catabolism control. To investigate this possibility, four Fe fr agments which differ in the presence/absence of hinge disulphides and hinge sequences have been analysed in both in vivo pharmacokinetic stu dies and in vitro by limited proteolysis with pepsin. The data show th at the presence of hinge disulphide(s) in the Fc fragment results in a longer intravascular half life but a higher susceptibility to pepsin attack. This, taken together with the knowledge that pepsin cleaves cl ose to the CH2-CH3 domain interface, suggests that the longer half lif e of disulphide linked Fc fragments relative to unlinked fragments may be due to conformational differences in this region of the IgG molecu le, and these conformational changes may affect the accessibility of t he catabolic site for binding to putative protective Fc receptors.