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Volume 271, Number 48, Issue of November 29, 1996 pp. 30311-30314
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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COMMUNICATION:
Heat-induced Conversion of Ovalbumin into a Proteinase Inhibitor

(Received for publication, September 10, 1996, and in revised form, October 3, 1996)

Philippe Mellet , Bernard Michels and Joseph G. Bieth

From the Laboratoire d'Enzymologie, INSERM Unité 392, Université Louis Pasteur de Strasbourg, F-67400 Illkirch, France and the  Laboratoire d'Ultrasons et de Dynamique des Fluides Complexes, CNRS URA 851, Université Louis Pasteur de Strasbourg, F-67070 Strasbourg, France

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 (K_i = 5 nM) and cathepsin G (K_i = 60 nM) and bovine chymotrypsin (K_i = 30 nM). I-ovalbumin also inhibits bovine trypsin, porcine elastase and -lytic proteinase with K_i 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 °C and 120 °C as tested by differential scanning calorimetry, and it retains full inhibitory capacity after heating at 120 °C. It has 8% less -helices and 9% more -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.

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