Heat-induced conversion of ovalbumin into a proteinase inhibitor

Ovalbumin is a member of the serine proteinase inhibitor (serpin) family but is unable to inhibit proteinases. Here we show that heating transforms it into inhibitory ovalbumin (I-ovalbumin), a potent reversible competitive inhibitor of human neutrophil elastase (Ki = 5 nM) and cathepsin G (Ki = 60 nM) and bovine chymotrypsin (Ki = 30 nM). I-ovalbumin also inhibits bovine trypsin, porcine elastase and alpha-lytic proteinase with Ki values in the micromolar range. Thus, I-ovalbumin differs from active serpins by its inability to form irreversible complexes with proteinases. I-ovalbumin is unusually thermostable: it does not undergo any structural transition between 45 degrees C and 120 degrees C as tested by differential scanning calorimetry, and it retains full inhibitory capacity after heating at 120 degrees C. It has 8% less alpha-helices and 9% more beta-sheet structures than native ovalbumin, as shown by circular dichroism. Our results show that the primary sequence of ovalbumin contains the information required for enabling the first step of the serpin-proteinase interaction to occur, i.e. the formation of the Michaelis-like reversible complex, but does not contain the information needed for stabilizing this initial complex.