Aggregation of porcine pancreatic phospholipase A2 and its zymogen induced by submicellar concentration of negatively charged detergents

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Abstract
The interaction of 2 sodium n-alkyl sulfates (C10 and C2) 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 .times. critical micelle concentration (cmc)] the enzyme (MW 14,000) is able to build up a complex with the detergent molecules having a MW of .apprx. 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 2-step process: First, 2 detergent molecules strongly bind to a hydrophobic surface region of the protein, previously called interface recognition site. Subsequently, at higher detergent concentrations suddenly 6 enzyme molecules aggregate, probably including .apprx. 30 additional detergent monomers. Although the zymogen of the pancreatic phospholipase A2 seems to form comparable high MW aggregates with these detergents, there are spectroscopic differences and higher detergent concentrations are required. Moreover, as will be shown in a following paper, only the phospholipase A2 becomes superactivated in these complexes.