Manipulate[ KmP = 117.6(* microM *); Km = 18(* microM *); k2 = 17.1(* s^-1 *); KmP = 117.6(* microM *); k2P =(*22.7 microMol/min/mg = *) 204.3(* 1/s *); En = 8.33(*microG/L*)/116483 1000; result0 = NDSolve[{ (* odes *) D[S[t], t] == -k2 En S[t]/(Km + S[t]), D[P[t], t] == A[t] k2P En M[t]/(KmP (1 + S[t]/Km)), S[t] + M[t] + P[t] == 100, (* S=FDG, M=FMG, P=flurescein *) A[t] == 0.22 S[t]^1.4 + 1, (* initil conditions *) P[0] == 0}, {S[t], P[t], M[t]}, {t, 0, 100}]; result = NDSolve[{ (* odes *) D[S[t], t] == -k2 (En0/116483*1000) S[t]/(Km + S[t]), D[P[t], t] == A[t] k2P En M[t]/(KmP (1 + S[t]/Km)), S[t] + M[t] + P[t] == S0, (* S=FDG, M=FMG, P=flurescein *) A[t] == 0.22 S[t]^1.4 + 1, (* initil conditions *) P[0] == 0}, {S[t], P[t], M[t]}, {t, 0, 100}]; Plot[{P[t] /. result, P[t] /. result0}, {t, 0, 100}, PlotRange -> Full, PlotStyle -> {{Blue}, {Red}}, AxesLabel -> {"Time (s)", "Fluorescein"}], {{S0, 100, "Initial Substrate (\[Mu]M)"}, 40, 200}, {{En0, 8.33, "\[Beta]-galactosidase (\[Mu]g/L)"}, 1, 16}]