For some enzyme reactions (for example, the hydrolysis of cAMP by phosphodiesterase in vertebrate retinal cones)the enzyme is present in large quantities,so that e0/s0 is not a small number.Fortunately,there is an alternate derivation of the Michaelis-Mentenrate equation that does not require that e= e0/s0 be small. Instead, if one or both of k-1and k2 are much larger than k1.e0, then the formation of complex c is a rapid exponential process,and can be taken to be inquasisteady state.Make this argument systematic by introducing appropriate nondimensional variables and then ?nd the resulting quasi-steady-state dynamics. (Segel,1988; Frenzen and Maini, 1988; Segel and Slemrod, 1989; Sneyd and Tranchina, 1989)