BIOLOGICAL THIOLS ELICIT PROLACTIN PROTEOLYSIS BY GLANDULAR KALLIKREIN AND PERMIT REGULATION BY BIOCHEMICAL PATHWAYS LINKED TO REDOX CONTROL

Rat glandular kallikrein (GK), a trypsin-like serine protease, cleaves rat prolactin (PRL) in vitro to novel forms detectable in vivo and likely to be of physiological significance. PRL proteolysis by GK is thiol-dependent, with thiols acting upon PRL to refold the molecule into novel conformations that are GK substrates. This study compared several natural and synthetic thiols for their ability to elicit PRL proteolysis by GK. Rat PRL was incubated with rat GK in the presence of various thiols and 0.5% Triton X-100, which enhances thiol-elicited proteolysis. Cleavage was analyzed by gel electrophoresis under reducing and nonreducing conditions. In the presence of Triton X-100, all low molecular weight thiols elicited PRL cleavage by GK. The order of potency was dithiothreitol > mercaptoethanol > lipoic acid > cysteamine = glutathione (GSH) = coenzyme A > cysteine. In the absence of Triton, however, dithiothreitol, coenzyme A, and mercaptoethanol were most effective in eliciting GK proteolysis. Triton X-100 enhanced PRL cleavage by 4-19-fold, depending upon the thiol used. Folding isomers of processed PRL observed following cleavage included disulfide-liked homodimers, oxidized monomers, reduced monomers and mixed disulfides; the folding isomers generated varied depending upon the thiol used. GSH potency in eliciting PRL proteolysis increased 10-fold in the presence of biochemical pathways shuttling reducing equivalents to GSH disulfide (GSSG). PRL cleavage by GK could be controlled by substrates, enzymes, and cofactors making up the reducing shuttle when GSSG was used. Thioredoxin (a protein disulfide oxidoreductase) potently elicited PRL proteolysis by GK in the presence of a reducing shuttle and Triton X-100. Thioredoxin was about 400 times more potent than GSSG under such conditions. The results document that biological thiols can elicit PRL proteolysis by GK and permit control of the reaction by biochemical pathways linked to redox control.