Posted by Sunnely on November 2, 2001, at 23:15:44
In reply to Re: Stomach Wants to Know: How LONG must I be empty?, posted by JGalt on November 1, 2001, at 23:05:54
> Cam, one thing I have to ask you about...you mention that grapefruit and the compound narnigen (contained in grapefruit) inhibit 3A4 enzymes in the liver. I had always previously thought that this was the case as well, but all the backed literature I found claimed that it almost completely only blocked that enzyme in the intestines. Any idea where we found/heard this about it being in the liver and whether or not there's any truth to it?
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Naringin is the main bioflavinoid in grapefruit juice. Naringin is not a potent cytochrome inhibitor, but is partially metabolized by intestinal bacteria to naringenin, which is a potent inhibitor of CYP450 enzymes, and was originally thought to be the component of grapefruit juice responsible for the interactions, although it was thought possible that another unidentified component in grapefruit juice may also have been responsible, since giving naringin alone does not seem to cause the same degree of inhibition as grapefruit juice.
Recently, researchers have isolated a group of compounds from grapefruit juice called furanocoumarins, which appear to be specific CYP3A4 inhibitors. A study of extracts of grapefruit juice interacting with rat and human CYP450 found that naringin accounted for only 10% of the inhibition of CYP activity seen with grapefruit juice. In vitro results show that a compound known as 6',7'-dihydroxybergamottin may be the chemical which accounts for the difference in effects of CYP3A substrates caused by grapefruit juice versus naringenin.
Recently, a study performed in cellular models has revealed that grapefruit juice significantly activates p-glycoprotein mediated reduction in bioavailability, partially counteracting the CYP3A4 inhibitory effects of grapefruit juice. This may explain why the effect of grapefruit juice on drug absorption is unpredictable and highly variable. P-glycoprotein is an efflux pump that, like CYP3A4 enzymes, is located at high levels in intestinal enterocytes, the primary site of oral absorption, where it actively secretes absorbed drug back into the gut lumen.
Reference: http://powernetdesign.com/grapefruit/general/mechanism.html
WHERE IS THE SITE OF CYP3A4 INHIBITION BY GRAPEFRUIT JUICE: LIVER OR GUT?
Grapefruit juice contains a number of natural substances, including furanocoumarin derivatives and others, that have the capacity to inhibit the activity of human cytochrome P450-3A (CYP3A) isoforms. Only CYP3A is affected to a clinically important extent - other CYP isoforms are not significantly affected, except possibly with very large quantities of grapefruit juice. Only the CYP3A isoforms present in the mucosal cells of the SMALL INTESTINE are inhibited by grapefruit juice. Hepatic (liver) CYP3A is affected to a small degree or not at all.
REQUIREMENTS TO GRAPEFRUIT JUICE-DRUG INTERACTIONS:
Drug interactions with grapefruit juice in general are unusual and will occur under well-defined circumstances.
1. The affected drug must be a substrate for metabolism by CYP3A.
2. The affected drug must have a low oral bioavailability due to extensive presystemic extraction (first-pass metabolism).
3. Biotransformation by CYP3A in the gastrotintestinal tract must account for a major component of this presystemic extraction.
4. An interaction with grapefruit juice is possible only when the affected drug is given orally. Parenterally (intramuscular or intravenous) administered drugs, even when they are CYP3A substrates, will not interact with grapefruit juice.
Reference: Greenblatt DJ, Patki KC, von Moltke LL, Shader RI: Drug interactions with grapefruit juice: An update (editorial). Journal of Clinical Psychopharmcology 2001;21(4):357-359.
poster:Sunnely
thread:82868
URL: http://www.dr-bob.org/babble/20011025/msgs/83046.html