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In the present study

ricin A chain（RTA）was first biotinylated

and then chemicallyintroduced to a monoclonal antibody against gastric cancer

MGb

2

to generate an immuno-ricin Achain conjugate（I-RTA）specific for gastric cancer.Subsequently

its endocytosis in human gas-tric cancer cell line SGC-7901 was investigated by double dynamic EM labeling technique utilizingstreptavidin-gold and sheep anti-mouse IgG-gold probes.In addition

the effects of verapamil onI-RTA endocytosis process were also observed.Our findings demonstrated that the main entryroute of I-RTA was non-coated microinvagination

followed by coated pits and interiorization ofmicrovilli.Intracellularly

the endocytosed I-RTA was quickly transported from tubulovesicularstrueutres to multivesicular bodies

and finally to lysosomes where it was degraded

while in thenon-membranous structures I-RTA was scanty.In the presence of verapamil

however

I-RTAwas found to stay longer in tubulovesicular structures and to move in a less amount and moreslowly into lysosomes.In the meantime

I-RTA was seen to be increased in the non-membranousstructures of the cytosol

and the in vitro killing efficacy was greatly augmented.