Michaelis-menten equation

Hi,

   We used known Vm and Km to simulate the data set (time, Cp) without
adding random error in there.  Yes, the line looks like very close
to a straight line.  But why can't we obtain the correct values with
fitting process?  We used optim first and then followed by using nls
to fit the model.  Thanks.

regards,

---Chun-ying Lee

Hmm, sorry, no. I'm talking through a hole in my head there.

Vm*y/(Km+y) makes OK sense. Vm is what you get for large y - passing
from 1st order to 0th order kinetics. However, looking at the data

 plot(PKindex)
 abline(lm(conc~time,data=PKindex))

shows that they are pretty much on a straight line, i.e. you are 
in the domain of 0-order kinetics. So why are you expecting the rate
of decrease to have changed by roughly 3/4 (from 2/3*Vm/Vd at y=2*Km
to 1/2*Vm/Vd at y=Km when you reach 4.67)??