Stereoselective sulfate conjugation of racemic 4-hydroxypropranolol by human and rat liver cytosol.
The objective of this study was to determine the stereochemistry of sulfoconjugation of a chiral phenolic amine drug, 4-hydroxypropranolol (HOP), by the human liver. The reaction was catalyzed by the 100,000 g cytosol as the phenolsulfotransferase (PST) enzyme source with PAP35S as the co-substrate. The enantiomers of the intact sulfate conjugate formed, (+)-HOP35S and (-)-HOP35S, were separated by HPLC and measured by liquid scintillation spectrometry. Complex velocity vs. substrate concentration curves were obtained with two peaks of activity, one at 3 microM (high affinity) and one at 500 microM (low affinity). The high-affinity reaction demonstrated a high degree of stereoselectivity. Whereas the affinity of the enantiomers for this reaction was identical, with a very low apparent KM value of 0.59 microM, the apparent Vmax value for (+)-HOPS formation was 4.6-fold higher than for (-)-HOPS. In sharp contrast, the low-affinity reaction, with an apparent KM of 65 microM, was not stereoselective. Inhibition of the high-affinity reaction by elevated temperature, but not by dichloronitrophenol, indicated that this activity was due to a monoamine form of PST. Inhibition of the low-affinity reaction by dichloronitrophenol, but not by elevated temperature, indicated that this activity was due to a phenol form of PST. As a comparison, experiments with the rat liver cytosol demonstrated only one activity, with apparent KM values of 50 microM for both enantiomers and opposite stereoselectivity in maximum velocity compared to humans, (+)/(-)-HOPS ratio 0.72. The results of this study demonstrate stereoselectivity in human hepatic sulfation of a chiral phenolic amine, with clear differences between PST isoenzymes.(ABSTRACT TRUNCATED AT 250 WORDS)
Walle
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Department of Cell and Molecular Pharmacology and Experimental Therapeutics, Medical University of South Carolina, Charleston 29425.
Walle
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UK
D000818:Animals; D015239:Arylsulfotransferase / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism*; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008297:Male; D010724:Phosphoadenosine Phosphosulfate / Q000378:metabolism; D010761:Phosphorus Radioisotopes; D011433:Propranolol / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics; D051381:Rats; D011919:Rats, Inbred Strains; D013237:Stereoisomerism; D013431:Sulfates / Q000378:metabolism*
D007527:Isoenzymes; D010761:Phosphorus Radioisotopes; D013431:Sulfates; C001422:4-hydroxypropranolol; D010724:Phosphoadenosine Phosphosulfate; D011433:Propranolol; D015239:Arylsulfotransferase