Both chaperone and isomerase functions of protein disulfide isomerase are essential for acceleration of the oxidative refolding and reactivation of dimeric alkaline protease inhibitor

Abstract Oxidative refolding of the dimeric alkaline protease inhibitor (API) from Streptomyces sp. NCIM 5127 has been investigated. We demonstrate here that both isomerase and chaperone functions of the protein folding catalyst, protein disulfide isomerase (PDI), are essential for efficient refolding of denatured‐reduced API (dr‐API). Although the role of PDI as an isomerase and a chaperone has been reported for a few monomeric proteins, its role as a foldase in refolding of oligomeric proteins has not been demonstrated hitherto. Spontaneous refolding and reactivation of dr‐API in redox buffer resulted in 45% to 50% reactivation. At concentrations <0.25 μM, reactivation rates and yields of dr‐API are accelerated by catalytic amounts of PDI through its isomerase activity, which promotes disulfide bond formation and rearrangement. dr‐API is susceptible to aggregation at concentrations >25 μM, and a large molar excess of PDI is required to enhance reactivation yields. PDI functions as a chaperone by suppressing aggregation and maintains the partially unfolded monomers in a folding‐competent state, thereby assisting dimerization. Simultaneously, isomerase function of PDI brings about regeneration of native disulfides. 5‐Iodoacetamidofluorescein–labeled PDI devoid of isomerase activity failed to enhance the reactivation of dr‐API despite its intact chaperone activity. Our results on the requirement of a stoichiometric excess of PDI and of presence of PDI in redox buffer right from the initiation of refolding corroborate that both the functions of PDI are essential for efficient reassociation, refolding, and reactivation of dr‐API.