Purine Nucleoside Phosphorylase (PNP) plays a crucial role in the biochemical pathways of nucleotide metabolism, specifically in the salvage pathway. It catalyzes the reversible phosphorolysis of purine nucleosides, such as inosine and guanosine, to their respective bases (hypoxanthine and guanine) and ribose-1-phosphate. This enzyme is essential for maintaining the balance of purine nucleotides within cells, which is vital for DNA and RNA synthesis.

In medical applications, PNP is a target for therapeutic interventions. Inhibitors of this enzyme are being explored for their potential in treating certain types of cancers and autoimmune diseases by disrupting the proliferation of rapidly dividing cells that rely heavily on nucleotide synthesis. Additionally, the enzyme itself is utilized in gene-directed enzyme prodrug therapy (GDEPT), where expression of bacterial PNP in tumor cells converts non-toxic purine nucleoside prodrugs into cytotoxic metabolites, enabling targeted killing of cancer cells.

Deficiencies in PNP activity are linked to severe combined immunodeficiency (SCID), a disorder characterized by impaired T-cell function. Understanding the enzyme's function and regulation can aid in developing gene therapy approaches to correct these deficiencies, thereby restoring immune function.

In biotechnology, PNP is employed in enzymatic assays and research to investigate nucleotide metabolism and its regulatory mechanisms. It also serves as a biocatalyst in the synthesis of nucleoside analogs, which are crucial for developing antiviral and anticancer drugs. Furthermore, it has applications in diagnostics for genetic disorders related to enzyme deficiencies.