Final phase of enzyme reactions following a Michaelis-Menten mechanisms in which the free enzyme and/or the enzyme-substrate complex are unstable.

An important kinetic analysis of unstable enzyme systems was carried out by Duggleby (Duggleby, R.G. (1986) J. Theor. Biol. 123, 67-80). This author states that his results are of general validity in the sense that the instability rate constants may have any value. Later, Wang & Tsou (Wang and Tsou (1990) J. Theor. Biol. 142, 531-549) rediscovered Duggleby's results when they analyzed a scheme in which the inactivations were due to a non-complexing irreversible inhibitor, pointing out the need to assume an initial steady-state in the catalytic route of the reaction. In the present contribution we show that there are values of the instability rate constants for which the equations of Duggleby are not applicable. We propose, for these cases, an alternative equation, which relates the final substrate concentration with the initial ones of both the substrate and the enzyme. Based on this, an experimental design for the evaluation of kinetic parameters is suggested. The present work concerns enzyme reactions evolving according to a Michaelis-Menten mechanism, in which the free enzyme and/or the enzyme-substrate complex are unstable.