ENDOCYTIC VESICLES IN LIVER CARRY POLYMERIC IGA FROM SERUM TO BILE

The distributions both of endogenous Ig[immunoglobulin]A and of injected 125I-labeled IgA were determined among the components of a liver homogenate. Rate zonal sedimentation, under conditions where separation was principally determined by particle size, showed that IgA was tightly bound to material which sedimented in the size range of the larger endoplasmic reticulum fragments. Further fractionation of the components within this size range according to their densities, by isopycnic centrifugation, showed that the IgA was associated with small vesicles with a density range of 1.12-1.17 g/ml, quite distinct from endoplasmic reticulum fragments. The IgA appears to be present in liver cells in a distinct class of vesicles, which are responsible for the transport of IgA from blood to bile.