Protein catabolism in cultured rat yolk sac

Abstract

An in vitro culture technique for the quantitation of pinocytosis
in the rat visceral yolk sac was modified, by removing calf serum from
the incubation medium, to permit determination of the rate of uptake of
substrates in the absence of competing serum proteins. Study of the
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Endocytic Index of I-labelled PVP in the absence of calf serum
indicated that the substrate provided a suitable marker for determination
of the rate of fluid ingestion in serum-free medium. From measurements
of the ratesof ingestion of [ U^^Cl sucrose, *2^I-labelled dBSA and
colloidal [ Aul gold in the presence and absence of calf serum, it was
concluded that the first substrate is ingested essentially in the fluid
phase, while the other two are ingested mainly adsorbed to the plasma
membrane. A detailed study of the uptake of I-labelled dBSA in the
presence of a non-radioactive analogue permitted a kinetic analysis of
the affinity of this substrate for binding sites on the plasma membrane.
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The rate of ingestion of homologous I-labelled IgG indicated that it
is ingested mainly in the adsorbed phase but at a slower rate than 125
I-labelled dBSA. On re-incubation of yolk sacs ’loaded’ in vitro with I-labelled IgG a significant percentage of the released radioactivity was macromolecular and tentatively identified as 125I-labelled IgG. This finding contrasted sharply with that for either 125I-labelled PVP, with which little radioactivity was released, or 125I-labelled dBSA, with which virtually all the radioactivity released was in a low molecular weight form. The observation that IgG can escape from the tissue intact is compatible with the suggestion that the rat yolk sac is instrumental in the transmission of passive immunity from mother to fetus prior to birth. A number of investigations were performed in an attempt to distinguish the mechanism of release. The findings suggest that release occurred by way of a separate transport vesicle as proposed by A. E. Wild.