Species differences in substrate specificity of pyrimidine nucleoside phosphorylase

Area of Science:

• Biochemistry
• Pharmacology
• Oncology

Background:

• Pyrimidine nucleoside phosphorylase (PNP) plays a key role in the metabolic activation of chemotherapeutic agents like 5-fluorouracil (5-FU).
• Understanding species-specific differences in PNP activity is critical for predicting drug efficacy and toxicity.

Purpose of the Study:

• To compare the activity of pyrimidine nucleoside phosphorylase (PNP) using uridine (Urd), deoxyuridine (dUrd), and thymidine (dThd) as substrates.
• To investigate PNP activity in neoplastic and normal tissues across human, rat, and mouse models.
• To determine the implications of observed PNP activity differences for 5-FU metabolism and clinical application.

Main Methods:

• Enzyme assays were performed using uridine, deoxyuridine, and thymidine as substrates.
• PNP activity was measured in both normal and neoplastic tissues from humans, rats, and mice.
• Comparative analysis of substrate utilization ratios between species was conducted.

Main Results:

• PNP activity was consistently higher in tumor tissues compared to normal tissues across all species studied.
• Significant differences in substrate preference were observed: rodents showed a Urd:dUrd:dThd activity ratio of approximately 10:7:1, while humans exhibited a ratio of 1:30:20.
• The primary PNP enzyme activity in rodents is Urd phosphorylase, whereas in humans, it is dThd phosphorylase.

Conclusions:

• Tumor PNP activity levels are a critical factor influencing the effectiveness of 5-FU.
• The distinct substrate specificities of human and rodent PNP necessitate the use of human tissues for accurate preclinical evaluation of 5-FU and its analogues.
• These findings highlight the importance of species-specific considerations in drug metabolism studies for optimizing cancer therapy.