HEAT-INDUCED CONVERSION OF OVALBUMIN INTO A PROTEINASE-INHIBITOR

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