Denaturation studies on natural and recombinant bovine prochymosin (prorennin).

1. Prochymosin in solution in the presence of 8 M-urea is fully unfolded, as indicated by its fluorescence spectrum, fluorescence quenching behaviour and far-u.v.c.d. spectrum. 2. Equilibrium studies on the unfolding of prochymosin and pepsinogen by urea were carried out at pH 7.5 and pH 9.0. The results indicate that the stabilization energies of the two proteins are identical at pH 7.5, but that at pH 9.0 pepsinogen is significantly less stable than prochymosin. 3. Kinetic studies on the unfolding of prochymosin and pepsinogen indicate that the processes can be described by a single first-order rate constant, and that at any given value of denaturant concentration and pH the rate of unfolding of prochymosin is significantly greater than that of pepsinogen. 4. Unfolding of prochymosin by concentrated urea is not fully reversible, unlike that of pepsinogen. Kinetic analysis of the refolding of the proteins suggests the presence of a slow process following unfolding in urea; for pepsinogen this process leads to a slowly refolding form, whereas for prochymosin the slow process in urea leads to a form that cannot refold on dilution of the denaturant. 5. The results provide a rationale for an empirical process for recovery of recombinant prochymosin after solubilization of inclusion bodies in concentrated urea. 6. In all respects studied here, natural and recombinant bovine prochymosin were indistinguishable, indicating that the refolding protocol yields a recombinant product identical with natural prochymosin.