Dear Mark, 

> 1). Does anyone know if mast cells posses any endogenous  biotin (a little or a lot?).
> 2). Does anyone know how to block endogenous biotin (apart from using a  commercial kit).

Bussolati and Gugliotta described binding of ABC to mast 
cells. They believe this to be due to both the binding of 
avidin basic residues as well as peroxidase to the sulphate 
groups of heparin. They showed that this can be prevented by 
using  the ABC solution at a pH of 9.4. This high pH does not 
affect previous binding of localisation antibody nor the 
affinity of biotin for avidin. They also showed that this 
nonspecific binding can also be blocked by a 30 minute 
pretreatment of sections with a synthetic basic polypeptide eg 
poly L lysine (0.01% in PBS pH 7.6).

Bussolati & Gugliotta (1983) J.Histochem.Cytochem., 31(12):1419-1421.

I hope this helps, 

Tony
Tony Henwood
Senior Scientist
Anatomical Pathology
Royal Prince Alfred Hospital
Sydney, AUSTRALIA

http://www2.one.net.au/~henwood
http://www.pathsearch.com/homepages/TonyHenwood/default.html