When studying urease at about the same time as Michaelis and Menten were studying invertase, Donald Van Slyke and G. E. Cullen made essentially the opposite assumption, treating the first step not as an equilibrium but as an irreversible second-order reaction with rate constant . As their approach is never used today it is sufficient to give their final rate equation:
and to note that it is functionally indistinguishable from the Henri–Michaelis–Menten equation. One cannot tell from inspection of the kinetic behaviour whether  is equal to  or to  or to something else.