Ultrastructural studies of the intracellular translocation of endocytosed alpha‐foetoprotein (AFP) by cytochemistry and of the uptake of 3H‐arachidonic acid bound to AFP by autoradiography in rat rhabdomyosarcoma cells

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Abstract
A covalent conjugate of alpha-foetoprotein (AFP) and horseradish peroxidase (HRP) has been used to follow, at the ultrastructural level, the pathway of AFP uptake and translocation in a rat rhabdomyosarcoma cell line. The cells were incubated for several times at 4°C and/or 37°C, and fixed. AFP-HRP was found to enter the cells via coated pits and receptosomes and to move to tubular elements of the trans-reticular portion of the Golgi. Some observations suggest that AFP can be recycled back to the cell surface. On the other hand, the cells were incubated with a noncovalent conjugate of AFP and 3H-arachidonic acid [3H-(20:4)], and the uptake of the fatty acid molecules studied by ultrastructural autoradiography. The cytoplasmic labeling, very low after an incubation in the presence of [3H-(20:4)]-AFP for 2 hours at 4°C, increased rapidly after transfer of the cells for 5 minutes to 37°C. These observations support the hypothesis that AFP plays a role in the intracellular delivery of polyunsaturated fatty acids.