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Publisher Summary Deoxythymidine is an intermediate in the degradation of thymine deoxynucleotides and of deoxyribonucleic acid (DNA). In most organisms, deoxythymidine is of minor importance for the biosynthesis of thymine deoxynucleotides and, therefore, for DNA synthesis. Deoxyuridine is formed as an intermediate in the degradation of deoxyuridine phosphates and by the deamination of deoxycytidine. Deoxythymidine and deoxyuridine occur only in very low concentration in biological material. Deoxythymidine is formed in large amounts in the chemical or enzymatic hydrolysis of DNA. The two nucleosides are usually characterized and determined by chromatography and by their UV absorption spectra. Deoxythymidine phosphorylase catalyzes a phosphorolytic cleavage of deoxythymidine and deoxyuridine to give deoxyribose-1-phosphate and the corresponding free base. The spontaneous hydrolysis of deoxyribose-1-arsenate proceeds at a rapid rate and therefore, the whole process is irreversible. The progress of the reaction in alkaline conditions can be measured by the increase in extinction at 300 or 290 nm. Deoxythymidine phosphorylase has a wide pH optimum between pH 5.8 and 6.8. The only physiological substrates for deoxythymidine phosphorylase are deoxythymidine and deoxyuridine.