Both alpha and beta subunits of human choriogonadotropin photoaffinity label the hormone receptor.

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Abstract
A photoactivable derivative of human choriogonadotropin (hCG) labels the lutropin receptor on porcine granulosa cells. To identify which of the hCG subunits labeled the receptor, 3 sets of different hCG derivatives were prepared. In the 1st set, hCG was coupled to the N-hydroxysuccinimide ester of 4-azidobenzoylglycine and radioiodinated. In the 2nd set, only one of the subunits was radioiodinated, but both subunits were allowed to react with the reagent. In the 3rd set, both the reagent and 125I were coupled to only one of the subunits. The binding activity of each hormone derivative was comparable to that of 125I-labeled hCG. After binding of these hormone derivatives to the granulosa cell surface, they were photolyzed. After solubilization, autoradiographs of sodium dodecyl sulfate/polyacrylamide gels of each sample revealed a number of labeled bands; the hCG derivatives containing 125I-labeled .alpha. subunit produced 4 bands (MW 120,000 .+-. 6000, 96,000 .+-. 5000, 76,000 .+-. 4000, and 73,000 .+-. 4000) and those containing 125I-labeled .beta. subunit produced 3 bands (MW 106,000 .+-. 6000, 88,000 .+-. 5000, and 83,000 .+-. 4000). Results were the same when the hormone-receptor complexes were solubilized in 0.5% Triton X-100, and then photolyzed, or when the hormone was derivatized with a family of reagents having arms of various lengths. Apparently both the .alpha. subunit and the .beta. subunit of hCG photoaffinity labeled certain membrane polypeptides, and these polypeptides are related to the hormone receptor.

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