HRP catalyzes an enzymatic reaction that yields an insoluble reaction product that can be made electron dense with the addition of osmium. Substrates in this reaction are hydrogen peroxidase and 3-3' diaminobenzidine (DAB,
Image 1 Cytochemical staining of yeast S. cerevisiae Sec7 cells expressing Hsp150Δ-HRP reveals Golgi elements (L. Karhinen).
Image 2 and 3 Cytochemically stained endoplasmic reticulum of NRK-52E cells expressing a construct where HRP is tagged with signal sequence and ER retention signal KDEL during interphase (upper, Jokitalo et al., 2001, JCB 154:317-330) or anaphase (lower, E. Jokitalo).
Image 4 Cytochemical staining of NRK-52E cells stably expressing Sialyl transferase-HRP chimera. The stain is present in the trans-cisternae (black arrow) and trans Golgi network (curved arrow; Jokitalo et al., 2001, JCB 154:317-330).
Image 5 HRP (e.g. Type II, Sigma P-8250) is added to the growth medium, and cells take it in by fluid phase endocytosis (pinocytosis). Cells can be fixed, or HRP-containing medium can be replaced with normal medium and HRP inside the cells is allowed to be transported further along the endocytotic pathway. Using different pulse-chase times, the HRP staining can be detected in early or late endosomes or lysosomes, or even in trans Golgi network. In this experiment, cells were incubated with HRP-containing medium for 15 minutes and fixed (O. Rämö & L. Mattinen, EM-course 2007).