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I'd just like to 2nd this query posed two years ago .... has anyone
determined scaling factors for pooled human liver S9 and cytosol,
i.e. ?mg fraction per g wet weight liver? I seem to be getting pretty
reasonable CLhepatic estimates for reference compounds using
Houston's recent (2006) 40mg protein per g liver estimate for human
liver microsomes. I'm now dealing with a series of compounds that
have both significant cytosolic and microsomal components to their
overall clearance ... hence the need to screen using S9 ..... but
without an accurate scaling factor I have no context for my data
(apart from rank order). Help!
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The following message was posted to: PharmPK
OK .... I'm talking to myself again!
Consider the following;
* "Generic" scaling factors for human liver microsomes have recently
been described. Housten (1) derived the following; 40mg microsomal
protein / g liver, from a multicenter study based on 38 livers (CV=30%).
This work included included a check on whether or not the CYP isoform
profile in the microsomes reflected that in the source liver homogenate
.... they were (i.e. the CYP isoforms were being recovered (roughly)
equally in the microsomes. Barter et al. (2) has collated data for 164
livers and arrived at; 35mg microsomal protein per liver (95% CI of the
mean = 33-36mg/g).
* Having in our posession supplies of pooled human liver microsomes
(PHLMs), pooled human liver S9 and pooled human liver cytosol that have
virtually identical donor profiles (i.e. sourced from the same livers)we
become extreemly keen to make some in vitro - in vivo predictions as
regards the relative contribution of microsomal and cytosolic enzymes to
the hepatic clearance of some of our NCEs.
* We derive the following factor; A = Microsomal Total P450 / S9 Total
P450. Where "A" represents an adjustment factor for PHLS9 CLint values
to enable them to be directly compared to PHLM CLint values
* After looking at a compoond "x" in PHLS9 and PHLMs we find that its
PHLS9 CLint * A > PHLM CLint we hypothesize that a cytosolic enzyme is
making a significant contribution to total CLint. Furthermore we then
carry out the simple calculation .... (PHLS9 CLint * A) - PHLM Clint
..... to get an estimate of x's PHLCyt CLint value. Could it really be
Best Regards, Gwyer.
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