BISTABILITY IN THE ISOCITRATE DEHYDROGENASE REACTION - AN EXPERIMENTALLY BASED THEORETICAL-STUDY

The enzyme isocitrate dehydrogenase (IDH, EC 1.1.1.42) can exhibit act ivation by one of its products, NADPH. This activation is competitivel y inhibited by the substrate NADP(+), whereas NADPH competes with NADP (+) for the catalytic site. Experimental observations briefly presente d here have shown that if IDH is coupled to another enzyme, diaphorase (EC 1.8.1.4), which transforms NADPH into NADP(+), the system can att ain either one of two stable states, corresponding to a low and a high NADPH concentration. The evolution toward either one of these stable states depends on the time of addition of diaphorase to the medium con taining IDH and its substrate NADP(+). We present a theoretical and nu merical analysis of a model for the IDH-diaphorase bienzymatic system, based on the regulatory properties of IDH. The results confirm the oc currence of bistability for parameter values derived from the experime nts. Depending on the total concentration of NADP(+) plus NADPH and th e concentration of IDH, the system can either admit a single steady st ate or display bistability. We obtain an expression for the critical t ime t, before which diaphorase addition leads to the lower steady sta te and after which addition of the enzyme leads to the upper steady st ate of NADPH. The analysis is extended to the case where the second su bstrate of IDH, isocitrate, is consumed in the course of the reaction without being regenerated. Bistability occurs only as a transient phen omenon in these conditions.