Solubilization and characterization of lactogenic hormone receptor from kidney of lactating cow

Solubilization of the microsomal fraction from bovine kidney by Triton X-100 or by 3-[(3-cholamidopropyl)-dimethylammonio] 1-propanesulfonate (CHAPS) increased 2-fold the thermodynamic association constant for hGH. While Solubilization with CHAPS did not change the 13-fold preferential binding of human growth hormone (hGH) over ovine prolactin (oPRL), solubilization with Triton X-100 increased this preference to 47-fold. The binding was optimal at pH 7-7.5 in the presence of 10 mM of MgCl₂ or CaCI₂. The association rate with hGH was identical in the microsomal and Triton X-100 solubilized fractions but the dissociation was slower in the latter. Only partial dissociation was observed at neutral pH. Full dissociation was, however, achieved by lowering the pH to 4-5, indicating that the binding was not covalent. Gel filtration studies of the Triton X-100 solubilized fraction after preincubation in the presence of reducing agent revealed two sharp peaks of activity, one having M_r of > 700 kDa that represented the aggregated receptor, and the second, with M_r. 110-115 kDa. The specificity of the partially purified receptors clearly shows that they are lactogenic and not somatogenic. They resemble lactogenic receptors found in other bovine organs, but differ from other species particularly in their differential affinities of PRL and hGH.