The digestion and transmission of labelled immunoglobulin G by enterocytes of the proximal distal regions of the small intestine of young rats.

Density gradient centrifugation of samples prepared from proximal gut homogenates after intra-lumenal injection of 125I-labelled IgG, was used to prepare batches of IgG fragments according to sedimentation coefficients. 2. Ultrafiltration was employed to partition the radioactivity in the vascular compartments, viscera and carcasses of rats aged 14-15 days, 2 hr after the injection of standard doses of labelled IgG into the proximal and distal regions of the small intestine. 3. Radioactive samples prepared by these methods were re-introduced into young rats by intra-cardiac injection, and the rate at which they were removed from the vascular compartment was assessed. 4. Proximal enterocytes transmitted about 39% of the IgG which had been removed from the intestine in intact form. Most of this was retained in the vascular compartment; they degraded up to about 57% of the total removed into fragments less than 1000 mol. wt. and about 4% into intermediate sized fragments. 5. Distal enterocytes degraded almost 90% of the IgG processed into fragments less than 1000 mol. wt., about 8% as fragments greater than 100,000 mol. wt. 6. Fragments, of all sizes, were cleared rapidly from the circulation into the viscera and carcass. 7. The relevance of these results to protein transmission and digestion by the rat small intestine is discussed.