Thymidine salvage changes with differentiation in human keratinocytes in vitro.

We compared the capacity of proliferating and differentiating keratinocytes to salvage and catabolize extracellular thymidine. Both populations of cells catabolized thymidine to thymine and possessed thymidine phosphorylase activity. As keratinocytes differentiate, thymidine phosphorylase activity ultimately increased twofold. In contrast, proliferating and differentiating keratinocytes differed markedly in their capacity to salvage extracellular thymidine. Proliferating keratinocytes readily salvaged extracellular thymidine to form nucleotides, whereas differentiating cells rapidly lost this capacity. The inability of differentiating cells to form nucleotides from thymidine was not attributed to reduced availability of thymidine due to catabolism but rather was the result of the rapid loss of thymidine kinase activity. As keratinocytes differentiate in suspension culture, they lose 41% of thymidine kinase activity in 8 h and over 90% of activity in 12 h. Our data indicate that loss of capacity to salvage extracellular thymidine for synthesis of nucleotides closely parallels the onset of differentiation in keratinocytes.