Retinol, a probe of conformational changes in protein disulfide isomerase

Nanosecond and steady fluorescence techniques have been employed to study the interaction of retinol with protein disulfide isomerase (PDI). Retinol binds tightly to PDI; and the rotational correlation time (θ = 36 ns) corresponds to a monomeric subunit of 55 kDa. The enzyme does not undergo aggregation in the presence of low molecular weight peptides. Under denaturing conditions; presence of 0.75 M Gnd HCl, the fluorescence yield of bound retinol is enhanced, suggesting stronger interactions of exposed hydrophobic groups of the protein with retinol. Based on far UV CD and fluorescence measurements of the protein in the presence of Gnd HCl, it is proposed the existence of molten globule intermediates during the unfolding of PDI.