PURIFICATION AND CHARACTERIZATION OF [H-3] MEPYRAMINE (HISTAMINE H-1 ANTAGONIST) BINDING-PROTEIN FROM RAT-LIVER - A HIGHLY HOMOLOGOUS PROTEIN WITH CYTOCHROME-P450 2D

A protein having a high-affinity binding site for [H-3]mepyramine (MBP ) was purified to homogeneity from rat liver membranes, The purified M BP has a single type of binding site for [H-3]mepyramine with K-d valu e of 18.5 nM, and its molecular weight was determined to be 56,000 by SDS polyacrylamide gel electrophoresis. Amino acid sequences of twelve tryptic peptides derived from MBP are highly homologous with those of rat debrisoquine 4-hydroxylase (cytochrome P450 2D1) and other rat P4 50 2D subfamily members, In immunoblotting analysis, an antibody again st rat P450 2D1 stained a band corresponding to MBP with M(r) of 56,00 0; its migration position was clearly different from that of rat P450 2D1. Substrates and inhibitors of debrisoquine 4-hydroxylase potently displace [H-3]mepyramine binding to MBP, Quinine and quinidine showed 400 and 80 times, respectively, higher affinity for MBP than for debri soquine 4-hydroxylase. These results suggest that MBP is a novel P450 2D family member.