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