Local and systemic implications of thymidine catabolism by human keratinocytes.

The human epidermis possesses a very active thymidine phosphorylase with the capacity to catabolize all of the thymidine available to the epidermis from the circulation and from the degradation of DNA in terminally differentiating keratinocytes. This high capacity of keratinocytes to catabolize thymidine could affect local levels of thymidine within the epidermis for DNA synthesis and could contribute to the regulation of the concentration of thymidine in the systemic circulation. Further work is needed to delineate the physiologic role of this keratinocyte enzyme. A practical consequence of the activity of epidermal thymidine phosphorylase is the role it may play in limiting the clinical efficacy of certain thymidine analogs, an important class of antiviral agents.