Whole protein uptake and metabolism by mouse blastocysts

Preimplantation mouse embryos take up whole 125I-labelled BSA from their environment. In blastocysts this uptake was temperature-sensitive and reversibly inhibited by trypan blue: properties consistent with an endocytotic mechanism. The uptake kinetics indicate that a saturable component predominates at low protein concentrations, but a non-saturable component is the major uptake route at higher concentrations. This suggests that BSA is pinocytosed probably bound to the membrane and dissolved in the bulk solvent phase. The rate of uptake, equivalent to about 5 pl/min/blastocyst was similar to that reported for non-saturable glycine uptake. In blastocysts the protein is degraded to acid-soluble products. At reported genital tract fluid protein concentrations this would represent a significant contribution to the embryonic pool of fixed nitrogen.