Isolation and characterization of native activin B.

To examine whether activin binds to follistatin, an activin-binding protein, to form a complex in vivo, we attempted to purify activin-follistatin complex from porcine follicular fluid. Our results thus obtained indicated that almost equimolar amounts of activins A, AB, and B are present as a complex with follistatin in the follicular fluid. Reverse-phase high performance liquid chromatography of the purified complex yielded follistatin and activins A, AB, and B. The activity of the purified activin B was found to be significantly lower than those of other activins in various assay systems such as stimulation of follicle-stimulating hormone secretion, induction of erythrodifferentiation, and potentiation of expression of gonadotropin receptors on ovarian cells. Moreover, binding of 125I-activin A to erythroleukemic cells which are activin-responsive was competed by activin B with approximately 10-fold lower potency compared with other activins. In contrast to these results, activin B was proved to have a potent Xenopus mesoderm-inducing activity, comparable with that of other activins. This indicates that, unlike activins A and AB, activin B can only elicit mesoderm-inducing activity and cannot function in other biological systems, suggesting a specific role of activin B in early development and unknown biological functions.