Transglutaminase activity in bovine calf testicular membranes: evidence for a possible role in the interaction of follicle-stimulating hormone with its receptor.

Transglutaminase (TGase) activity, determined by incorporation of [1,4-14C]putrescine into casein, was demonstrated in a light membrane fraction derived from bovine calf testicular homogenates. In common with other TGases thus far identified, testicular TGase is calcium dependent. A concentration-dependent relationship was observed between decreasing TGase activity and increasing concentrations of N-ethylmaleimide and bacitracin, inhibitors of TGase, and the TGase substrate monodansylcadaverine. Although NEM at all concentrations tested had no effect on [125I]human (h) FSH-receptor binding, dissociation of [125I]hFSH from its receptor was enhanced when [125I]hFSH-receptor complexes were formed in the presence of NEM. Dissociation of [125I]hFSH from its receptor also increased in the presence of bacitracin or monodansylcadaverine. Reduced TGase activity always paralleled increases in hormone-receptor dissociation. The inverse relationship between TGase activity and [125I]hFSH-receptor dissociation observed in this study suggests that TGase activity may be involved in the interaction of FSH with its receptor and that protein cross-linking (via peptide bond formation) may be a mechanism whereby some FSH-receptor complexes are stabilized in the bovine testis.