EXISTENCE OF PHOSPHORYLATED AND DEPHOSPHORYLATED FORMS OF CYTOSOLIC THYMIDINE KINASE (TK1)

In this study we examine whether different TK1 variants of pI 6.9 and 8.3 found by isoelectric focusing gel electrophoresis (IFE) reflect ju st a phenotype difference due to phosphorylation modifications or have a real phenotypic background. The phosphorylation degree of purified TK1 variants was analyzed by determining the changes in the pI values after treatment with alkaline phosphatase, using IFE. The genetic orig in of the two TK1 variants was studied by determining their mol wt. by means of SDS-gelelectrophoresis. Furthermore, the subcellular distrib ution of the two TK1 variants was also studied. Alkaline phosphatase t reatment changed the pI value of purified TK1 from 6,9 to 8.3. No chan ge in the pI value was found when purified TK1 corresponding to pI 8.3 was treated in the same way. Similar results were obtained when treat ed a cytosolic fraction with alkaline phosphatase. Antibody raised aga inst the C-terminal part of human TK1 only recognized the dephosphoryl ated TK1 variant corresponding to pI 8.3. There was no difference in t he molecular weight between the two TK1 variants. Thus, we concluded t hat the TK1 variants corresponding to pI 6.9 and 8.3 are of the same g enetic origin, but consist of phosphorylated and dephosphorylated form s.