Guinea pig eosinophil major basic protein (GMBP) as a potent histamine releaser. (I) Histamine releasing activity of GMBP and its chemical structure

When isolated guinea pig major basic protein (MBP) was applied to SDS-PAGE, it exhibited a single band with an apparent molecular weight of 11000. When rat peritoneal mast cells were exposed to MBP at concentrations higher than 0.3 M, significant histamine release was elicited both in the presence and absence of extracellular Ca²⁺. The histamine releasing activity of MBP was more pronounced in a Ca-free medium than in the presence of Ca²⁺. When MBP was applied on reverse phase HPLC, two distinct peaks were observed. These two proteins exhibited identical molecular weights and pI values. Their amino acid compositions were very similar. It was therefore assumed that guinea pig MBP is composed of two subtypes, namely GMBP₁ and GMBP₂, and that both have similar histamine releasing activities. MBP elicited not only⁴⁵Ca uptake from the extracellular medium but also induced Ca²⁺ release from intracellular Ca stores.