[R] Michaelis-menten equation
p.dalgaard at biostat.ku.dk
Wed Jul 20 10:50:18 CEST 2005
"Chun-Ying Lee" <u9370004 at cc.kmu.edu.tw> writes:
> 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.
Check your simulation. There is no way that curve is consistent with
Km in the middle of the y range!
> ---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)??
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O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
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