RETINOL FREE AND RETINOL COMPLEXED BETA-LACTOGLOBULIN BINDING-SITES IN BOVINE GERM-CELLS

A high affinity specific binding site for bovine beta-lactoglobulin (B LG) was identified in bovine germ cell plasma membrane enriched fracti ons. Binding was found to be reversible and pi-I-dependent with maximu m binding occurring at pH 5. The on-rate and off-rate constants were 2 .26 +/- 0.8 x 10(5) M-1 min(-1) (n = 3) and 0.016 +/- 0.004 min(-1) (n = 3), respectively. Scatchard analysis showed a single class of bindi ng sites, with 12.38 +/- 4.62 x 10(12) sites per mg of membrane protei n (n = 3) and a dissociation constant (K-D) estimated at 26.43 +/- 2.6 8 nM. There was inhibition of iodinated-BLG (variant A) (I-125-BLGA) b inding to germ cell plasma membrane enriched fractions in the presence of unlabelled BLG variant A, BLG variant B, retinol complexed BLGA an d human retinol-binding protein. Inhibition was observed neither with BSA nor with lactoferrin. I-125-BLGA incubated with a Triton X-100 sol ubilized plasma membrane fraction formed a high molecular mass complex in Superose 12B gel filtration. This receptor complex disappeared in the presence of unlabelled BLGA and in the presence of 10 mM EDTA. The results suggest that germ cell plasma membrane may contain a receptor which is capable of binding either retinol free or retinol complexed BLGA.