Substrate-specific Metabolism Research Articles

Incorporation of Ambient Water-H into the C-Bonded H Pool of Bacteria during Substrate-Specific Metabolism

The stable isotope ratios of C-bonded H (δ2Hn values) can be used to locate soil samples for forensic purposes because of their close correlation with the δ2H values of precipitation. Post-sampling bacterial activity might change the δ2Hn values via glycolysis. We tested to which degree C-bonded H is replaced by H from ambient water under favorable and unfavorable growing conditions. We provided two heterotrophic bacteria (Bacillus atrophaeus, Escherichia coli) with glucose (favorable) or lysine (unfavorable) under aerobic conditions. We assessed the H incorporation from ambient water via 2H labeling. We found that the H incorporation into bacterial biomass in the glucose treatment was 79 ± 5.9% (B. atrophaeus) and 43 ± 3.0% (E. coli), likely as a consequence of glycolysis and conservation of the δ2H value in the anabolic mode of the tricarboxylic acid (TCA) cycle. Differences between species were possibly related with different compositions of metabolite mixtures. The bacteria did hardly grow with lysine while respiration continued, and we found no H incorporation because the catabolic mode of the TCA cycle, which was active when the bacteria grew on lysine, is associated with CO2 release and a complete cleavage of former C–2H bonds. Our results support the glycolysis pathway as a mechanism underlying the incorporation of ambient-water H into the C-bonded H pool of bacteria. Stressful conditions forcing bacteria into a catabolism-dominated metabolism disable the incorporation of ambient-water H, and δ2Hn values can be applied to identify the origin of soil samples in forensics.

Substrate specific metabolism by polymorphic cytochrome P450 2D6 alleles

A comparative metabolism study was performed for bufuralol, dextromethorphan, imipramine, mianserin, sparteine, tamoxifen, haloperidol and two drug candidates (Rec27/0110 and Rec15/2739) on V79 cells genetically engineered to express human cytochrome P450 (CYP) variants 2D6*1, 2D*2, 2D*9 and 2D*17. Unexpectedly, the CYP2D6*17 dependent metabolism profile of haloperidol and Rec27/0110 were found to differ from all other substrates tested. Some of these known standard substrates are frequently applied in marker reactions for CYP2D6 and with these standard substrates, CYP2D6*1 is known to be the most active form. In both cases of haloperidol and Rec27/0110 the variant form CYP2D6*17 had equal or higher activity compared to the CYP2D6*1 form. Results obtained with the V79 cells were confirmed using microsomal preparation of yeast cells expressing the variants CYP2D6*1 and CYP2D6*17 and CYP2D6 inhibitor quinidine. In conclusion, there is no general rule for a variant dependent metabolism profile by cytochrome P450 2D6 indicating that the activity profile of the CYP2D6 alleles may be substrate specific, thus may be reflected in pharmacokinetics consequences for individuals.

Recent Developments in Drug Metabolism of Relevance to Psychiatrists

The discovery in 1977 of stable genetic differences (polymorphisms) in the oxidative metabolism of debrisoquine (P450 2D6) has led to an increasingly rapid growth in knowledge concerning the diversity of cytochrome P450 enzymes, their substrate-specific metabolism, and their interindividual variation. The current state of this knowledge for cytochromes P450 2D6, 2C19, 1A2, 3A3, and 2E1 is summarized, with a view toward their potential relevance to clinical psychopharmacology. To date, no prospective trials have tested the application of clinical tests of drug metabolism to treatment with psychotropics, and no guidelines have been formulated for such an application. The author reports preliminary findings from his studies of P450 2D6 phenotyping in elderly patients treated with nortriptyline and perphenazine. Research in this area may lead to earlier identification of patients at risk because of atypical drug metabolism. This might reduce drug-drug interactions, overmedication, or inadequate treatment.