PHOTOAFFINITY-LABELING WITH ONE-11-ALPHA-HEMISUCCINATE(2-[I-125]IODOHISTAMINE) IDENTIFIES 4 PROTEIN BANDS IN MOUSE-BRAIN MEMBRANES

The radiolabeled progesterone (PG) analogue progesterone-11 alpha-hemi succinate-(2-[I-125]iodohistamine) was used to label PG binding protei ns in brain membranes from mouse cerebellum. Photoaffinity labeling an d sodium dodecyl sulfate-polyacrylamide gel electrophoresis identified specific PG binding protein bands 1-4 of 64-29 kDa. Bands 1 and 4 wer e well resolved on the gel and easily quantified. Preincubation with P G inhibited photolabeling in a dose-dependent manner. The labeling was specific with respect to steroid structure. For band 1, the extent of inhibition of labeling by PG and 3 alpha,5 alpha-pregnanolone (3 alph a) was pronounced. Other steroids such as testosterone (Tes), estradio l (Est), and corticosterone (Cor) were less effective, whereas pregnen olone sulfate (PS) and cholesterol (Cho) were ineffective. With respec t to band 4, Est was the most effective; PG, 3 alpha, and Tes were int ermediate; and PS, Cho, and Cor were ineffective. The results describe specific membrane proteins that bind PG (band 1) and Est (band 4).