Re: [R] Michaelis-menten equation

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Wed 20 Jul 2005 - 18:50:18 EST

"Chun-Ying Lee" <u9370004@cc.kmu.edu.tw> writes:

> 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,

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             ุster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)                  FAX: (+45) 35327907

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Received on Wed Jul 20 18:56:06 2005

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