The 14CO2 breath test: facilities and limitations of a rapid and noninvasive method for in vivo evaluation of modified hepatic cytochrome P-450--a critique.

By means of the breath test technique the cascade from O-demethylations to CO2 was investigated after pretreatment of mice with warfarin, phenobarbital, cobaltous chloride, sodium vanadate and metyrapone. It was the intention to examine the validity of the technique with respect to cytochrome P-450 activity. Therefore three different radioactive labeled substrates, i.e., hydrogen carbonate, formate and xenobiotics, were applied at three different levels of the one-carbon pathway and were utilized to demonstrate possible interference of the modifiers with the sequence from O-demethylation to CO2. Real in vivo information about a modified cytochrome P-450 system can be obtained using model substrates carefully selected with regard to the type of expected modification of the monooxygenase system. In addition, a parallel monitoring of the consecutive reaction sequence by measuring the conversion of formate to CO2 is necessary in order to guarantee the validity of the in vivo technique in visualizing the activity of the hepatic monooxygenase system.