{ diff(x1, t) = (1*k8*k1*x1*x2/(k2 + x2) + (-1)*k8*(k3*x1*x2/(k4 + x1) + 1/k5*x1))/k8,
  diff(x2, t) = (1*k8*k6*x2 + (-1)*k8*k7*x1*x2)/k8 }