DPSgt & endocytosis

[Ruben Donis]

Dear fellow netter:
Bretscher et al. described a neat reagent to detect endocytosed proteins. 
Does anyone know if this reagent is available commercially (or otherwise) ?
Here is the reference:
M. S. Bretscher and R. Lutter, 1988, A new method for detecting endocytosed
proteins.  Embo J 7: 4087-92
--------- A new reagent, DPSgt, is described which has been designed to
label cell surface proteins at 0 degree C. The reagent is easily made; it
is water soluble and contains a reactive impermeant ester at one end, a
tyrosine which can be radioiodinated at the other, and a disulphide
in-between. The label can be removed from cells by cleaving the disulphide
linkage in it with glutathione at 0 degree C. When cells are warmed to 37
degrees C between labelling and reduction, labelled proteins which are
endocytosed acquire resistance to reduction. This provides a simple way of
measuring the endocytosis of surface proteins. The intracellular pools of
transferrin and LDL receptors in K562 cells and fibroblasts have been
estimated. The results indicate that intracellular receptors are in
non-reducing compartments, and that uptake of average cell surface (by
non-coated pit processes) in K562 cells is small.
-----
Any info greatly appreciated on this or similar reagent. 
Cheers,
Ruben
---------
Dr. Ruben Donis                                                 
Dept. of Veterinary and Biomedical Sciences  
202 VBS                                                                    
     
University of Nebraska,                                                  
Lincoln, NE 68583-0905                                   
Phone: 402-472-6063                                     
FAX to 402-472-9690                                     
E-mail RDONIS at UNL.EDU