Toxicological significance of dog liver cytochrome P-450: examination with the enzyme expressed in Saccharomyces cerevisiae using recombinant expression plasmid

A variety of cytochrome P450 enzymes and flavin-containing monooxygenases in dogs and pigs commonly used as preclinical animal models

Drug oxygenation is mainly mediated by cytochromes P450 (P450s, CYPs) and flavin-containing monooxygenases (FMOs). Polymorphic variants of P450s and FMOs are known to influence drug metabolism.　Species differences exist in terms of drug metabolism and can be important when determining the contributions of individual enzymes. The success of research into drug-metabolizing enzymes and their impacts on drug discovery and development has been remarkable. Dogs and pigs are often used as preclinical animal models. This research update provides information on P450 and FMO enzymes in dogs and pigs and makes comparisons with their human enzymes. Newly identified dog CYP3A98, a testosterone 6β- and estradiol 16α-hydroxylase, is abundantly expressed in small intestine and is likely the major CYP3A enzyme in small intestine, whereas dog CYP3A12 is the major CYP3A enzyme in liver. The roles of recently identified dog CYP2J2 and pig CYP2J33/34/35 were investigated. FMOs have been characterized in humans and several other species including dogs and pigs. P450 and FMO family members have been characterized also in cynomolgus macaques and common marmosets. P450s have industrial applications and have been the focus of attention of many pharmaceutical companies. The techniques used to investigate the roles of P450/FMO enzymes in drug oxidation and clinical treatments have not yet reached maturity and require further development. The findings summarized here provide a foundation for understanding individual pharmacokinetic and toxicological results in dogs and pigs as preclinical models and will help to further support understanding of the molecular mechanisms of human P450/FMO functionality.

The in vivo tyrosine phosphorylation level of yeast immunophilin Fpr3 is influenced by the LMW-PTP Ltp1

Tyr-phosphorylation in Saccharomyces cerevisiae is essential in controlling the activity of MAP kinase regulating mating, pseudohyphal growth, and cell wall biosynthesis. Yeast serves as a model system for studying the biological function of many protein kinases and PTPs. Two LMW-PTP from yeast have been cloned, namely, Ltp1 from S. cerevisiae and Stp1 from Schizosaccharomyces pombe. The sequences of both enzymes are relatively similar to those of the mammalian LMW-PTP. Recently we showed that the yeast immunophilin Fpr3 interacts with Stp1 and its dephosphorylated state induces a growth defective phenotype. Here we show the phosphatase activity of Ltp1 on Fpr3 and we demonstrated that Tyr 184 is the residue phosphorylated on in vivo Fpr3. We also described the marked activation of Ltp1 by adenine in S. cerevisiae proteome and determined in vivo the influence of tyrosine phosphorylation on Fpr3 localization.

Aryl hydrocarbon receptor-mediated suppression of expression of the low-molecular-weight prekininogen gene in mice

Differential mRNA display showed that a cDNA band disappeared after treatment of mice with 3-methylcholanthrene (MC). The cDNA encoded low-molecular-weight (LMW) prekininogen, known to be the precursor of a potent vasodilator, bradykinin. MC is generally known to bind to aryl hydrocarbon receptor (AhR) as an initial event to cause effects in vivo. In accordance with the results, Northern blot analysis for LMW prekininogen mRNA using total RNAs from wild-type and AhR-null mice indicated that the suppression of the mRNA expression by MC was seen in wild-type mice but not in AhR-null mice. The expression of LMW prekininogen mRNA was almost completely lost within 1 h after treatment of mice with MC, while a clear increase of CYP1A2 mRNA, as a positive control, was noted 4 h after the treatment. The plasma concentration of bradykinin released from LMW prekininogen was decreased by MC in wild-type mice, but not in AhR-null mice. Based on these results, we conclude that AhR inhibits bradykinin synthesis in mice via suppression of the expression of LMW prekininogen. Possible mechanism(s) responsible for hypertension caused by treatment of mice with MC is also discussed.

Pharmacokinetics and pharmacodynamics in toxicology

1. Pharmacokinetics aids interpretation of the dose-response relationship in individual toxicology studies. 2. When used to compare across studies, even in a single species other factors, including variation in pharmacodynamic response, must be taken into account. Variation in pharmacodynamic response becomes more profound when one compares across species. 3. Examples do occur where plasma concentration-response relationships are constant across species, particularly when corrected for unbound drug. These examples should not be taken as support, however, of a general universal principle. 4. Owing to multiple factors such as species differences in receptors, enzymes and ion channels, dose or plasma concentration-response relationships can vary enormously across species. In the light of this, the results of toxicology studies should be viewed as qualitative rather than quantitative. Once sufficient clinical experience is gained the human database is the overriding measure of drug safety.