CIRCULAR-DICHROISM OF PORCINE, BOVINE, AND EQUINE PANCREATIC PHOSPHOLIPASES-A2 AND THEIR ZYMOGENS - UNUSUAL CONFORMATIONS SIMULATING HELIX CONTENT

Conformation of porcine, bovine, and equine pancreatic phospholipases A2 (EC 3.1.1.4) and their zymogens was studied by the circular dichroism (CD) probe in the far and near UV spectral zones. All these phospholipases and their zymogens displayed CD curves suggesting the presence of moderate amounts of .alpha.-helical conformation. On the basis of known primary structure and recent X-ray structural analysis of prophospholipase A2 crystals it had to be concluded that the positive CD band centered at 190-191 nm and the negative bands located at 208-210 nm and 222-225 nm, respectively, was caused not only by .alpha.-helics but also by other symmetric structures, probably rings containing disulfide bonds. The main chain conformation of the phospholipases and their zymogens was found very resistant to acid but it was sensitive to sodium dodecyl sulfate in acid solutions and, to a lesser extent, to alkali. According to the observed CD changes in the presence of perturbants, the stability of the macromolecules of these proteins was determined chiefly by the difulfide bonds and hydrophobic interactions. The CD spectra in the near UV zone showed that the differences between the enzymes and their zymogens, with respect to the tertiary structure, were very small. Also it was observed that the tertiary structure of the horse phospholipase differed from the tertiary structure of the porcine and bovine phospholipases. The tertiary structure of all these enzymes and zymogens was very sensitive to sodium dodecyl sulfate.