Biological and immunological characterization of cleaved and 16K forms of rat prolactin

A cleaved form of rat PRL (rPRL) has been reported to exist in the pituitary gland and to be the main product of PRL proteolysis by its target tissues. A 16K fragment derived from cleaved PRL (cPRL) has mammary mitogenic activity in rats in vivo, and we found that both cleaved and 16K PRLs are capable of binding to hepatic PRL receptors. To analyze the functional significance of cleaved and 16K PRLs, ample amounts of cPRL were generated by incubation of 24K rPRL in vitro with a 25,000 X g rat mammary gland pellet. cPRL was reduced (2-mercaptoethanol), and the 16K and 8K fragments were separated by gel filtration. The purity of the cleaved and 16K samples was established by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their mitogenic [pigeon crop-sac, Nb2 lymphoma cell, and mammary cell bioassays (BAs)], lactogenic (casein BA), and immunologic (PRL RIA) activities were determined. cPRL had the same mitogenic activity, but half the immunoactivity, as intact PRL. The 16K fragment was biologically active in all assays. Its estimated mitogenic and lactogenic potencies were 65% and 10% those of the 24K PRL, respectively, and it had only 2% the immunoactivity of the intact hormone. Thus, the 16K fragment had higher BA to RIA ratios than cleaved or intact 24K PRLs. The functional significance of 16K PRL may have been underestimated owing to its low RIA activity.