Binding and degradation of doubly radioiodinated luteinizing hormone by mouse Leydig cells

Doubly radioiodinated LH (**LH) was used to examine the fate of both subunits during interaction with freshly isolated Leydig cells. Cells that had bound **LH and were then resuspended in **LH-free medium released radioactivity continuously from both subunits in acid-soluble form, but to only a limited extent in acid-precipitable form. With cells from BALB/c mice, the initial rate of release of acid-soluble radioactivity was substantially greater from alpha-subunit than from beta-subunit; this difference was not apparent with cells from Swiss-Webster mice. Appearance of acid-soluble radioactivity was inhibited by leupeptin; testosterone production was not affected. Cell-associated radioactivity declined when the resuspension medium contained unlabeled LH, but assumed a steady state when cells were incubated continuously in **LH. Thus, upon binding of LH to receptor, both subunits are internalized and degraded within the lysosome. Binding and degradation can proceed simultaneously, yet independently. LH degradation has no role in acute testosterone production.