Identification of protein disulfide isomerase as a phorbol ester-binding protein

A specific binding protein for 12-O-tetradecanoylphorbol 13-acetate (TPA), different from protein kinase C (PKC) and histone H1, was purified from HeLa cell extract by the use of affinity gel pendanted with phorbol ester (TPA-GEL). The purified binding protein was identified as protein disulfide isomerase (PDI, EC 5.4.3.1) by peptide sequence analysis. The dissociation constants (Kd's) of TPA to PDI, histone H1 and PKCalpha were determined to be 1.03 x 10(-6) M, 5.70 x 10(-7) M, and 4.00 x 10(-7) m, respectively, by the surface plasmon resonance (SPR) method. TPA moderately inhibited PDI activity assessed in terms of reactivation of denatured RNase A.