{ diff(x1, t) = ((-1)*k23*(k1*x1 + k2*x5*x1/(k3 + x1)) + 1*k23*k17 + (-1)*k23*k19*x1)/k23,
  diff(x2, t) = (1*k23*(k1*x1 + k2*x5*x1/(k3 + x1)) + (-1)*k23*(k4*x2*x9 - k5*x3) + 1*k23*k15*x6 + 1*k23*k6*x5 + (-1)*k23*(k11*x2*x8 - k12*x10))/k23,
  diff(x3, t) = (1*k23*(k4*x2*x9 - k5*x3) + (-1)*k23*(k9*x3 - k10*x4))/k23,
  diff(x4, t) = (1*k23*(k9*x3 - k10*x4) + (-1)*k23*k7*x13*x4/(k8 + x4))/k23,
  diff(x5, t) = (1*k23*k7*x13*x4/(k8 + x4) + (-1)*k23*(k16*x5*x8 - k12*x6) + (-1)*k23*k6*x5)/k23,
  diff(x6, t) = (1*k23*(k16*x5*x8 - k12*x6) + (-1)*k23*k15*x6)/k23,
  diff(x7, t) = (1*k23*k15*x6 + 1*k23*k6*x5)/k23,
  diff(x8, t) = ((-1)*k23*(k16*x5*x8 - k12*x6) + 1*k23*k15*x6 + (-1)*k23*(k11*x2*x8 - k12*x10) + 1*k23*k13*x10*x11/(k14 + x10) + 1*k23*k21 + (-1)*k23*k22*x8)/k23,
  diff(x9, t) = ((-1)*k23*(k4*x2*x9 - k5*x3) + 1*k23*k18 + (-1)*k23*k20*x9)/k23,
  diff(x10, t) = (1*k23*(k11*x2*x8 - k12*x10) + (-1)*k23*k13*x10*x11/(k14 + x10))/k23,
  diff(x11, t) = 0/k23,
  diff(x12, t) = 1*k23*k13*x10*x11/(k14 + x10)/k23,
  diff(x13, t) = 0/k23 }