Immunoreactive beta-endorphin in the rat mammotropic transplantable tumor (MtT-F4).

Raised levels of what appeared to be beta-lipotropin (beta-LPH), beta-melanotropic hormone, and beta-endorphin were detected by radioimmunoassay in the plasma of rats bearing the mammotropic transplantable pituitary tumor MtT-F4. The immunoreactivity to anti-beta-endorphin in the assay was displayed by a substance with the molecular weight of beta-LPH, as determined by gel filtration. Isolated cells of MtT-F4 tumor incubated in vitro released immunoreactive beta-LPH and beta-endorphin, with the expected molecular weights, into the incubation medium. These results suggest that the pituitary transplantable rat tumor MtT-F4 secretes peptides structurally related to beta-LPH.