Aggregation of porcine pancreatic phospholipase A2 and Its zymogen induced by submicellar concentrations of negatively charged detergents.

Abstract

The interaction of two sodium n-alkyl sulfates (C10 and C12) with porcine pancreatic phospholipase A2 and its zymogen was studied by various spectroscopic techniques, equilibrium gel filtration, calorimetry, and photochemically induced dynamic nuclear polarization 1H NMR. At very low concentrations of n-alkyl sulfate [up to 0.07 X critical micelle concentration (cmc)] the enzyme (Mr 14 000) is able to build up a complex with the detergent molecules having a molecular weight of about 90 000. This complex consists of 6 enzyme molecules and about 40 n-alkyl sulfate monomers. The formation of the detergent-protein aggregate occurs in a two-step process: First, two detergent molecules strongly bind to a hydrophobic surface region of the protein, previously called interface recognition site [Pieterson, W. A., Vidal, J. C., Volwerk, J. J., & de Haas, G. H. (1974) Biochemistry 13, 1455-1460]. Subsequently, at higher detergent concentrations suddenly 6 enzyme molecules aggregate, probably including about 30 additional detergent monomers. Although the zymogen of the pancreatic phospholipase A2 seems to form comparable high molecular weight aggregates with these detergents, there are spectroscopic differences, and higher detergent concentrations are required. Moreover, as will be shown in the following paper [Hille, J. D. R., Egmond, M. R., Dijkman, R., van Oort, M. G., Sauve, P., & de Haas, G. H. (1983) Biochemistry (following paper in this issue)], only the phospholipase A2 becomes superactivated in these complexes.