Sensitive nonradiometric method for determining thymidine kinase 1 activity.
BACKGROUND
Thymidine kinase 1 (TK1) is a cytoplasmic enzyme, produced only in the S-phase of proliferating cells, that has potential as a tumor marker. Specific determination of TK1 in serum is difficult, in part because of differences in the physical properties of serum TK1 compared with cytoplasmic TK1.


METHODS
The first step in the new assay was phosphorylation of 3'-azido-2',3'-deoxythymidine (AZT) to AZT 5'-monophosphate (AZTMP) by TK1 present in patient material. The AZTMP formed was measured in a competitive immunoassay with specific anti-AZTMP antibodies and AZTMP-labeled peroxidase. Results were compared with those of a TK radioenzyme assay (REA) for 78 samples from patients suffering from hematologic diseases.


RESULTS
The detection limit was 78 microIU/L, and within-run CVs <20% were seen for samples with TK1 down to 130 microIU/L. Cross-determination of the mitochondrial isoenzyme TK2 activity was <0.1%. Between-assay imprecision (CV) was 3.5-7.4%, and the within-assay imprecision was 4.1-9.1%. In studies of recovery and linearity on dilution, measured values ranged from 84% to 115% of expected at concentrations of 0.26-10.4 mIU/L. Results of the new assay (mIU/L) = 0.109 x TK REA (U/L) + 0.092. Heterophilic antibodies did not interfere in the assay. The upper 95th percentile, in 100 healthy individuals, was 0.94 mIU/L, and the median value was 0.43 mIU/L.


CONCLUSIONS
The TK1 enzyme-labeled immunoassay uses a stable substrate, is precise, appears to be accurate, and is resistant to interferences. It may provide a practical tool in the management of hematologic malignancies.
Ohrvik
Anders
A
DiaSorin AB, SE-161 02 Bromma, Sweden.
Lindh
Maria
M
Einarsson
Roland
R
Grassi
Jacques
J
Eriksson
Staffan
S
D014408:Biomarkers, Tumor / Q000097:blood*; D001943:Breast Neoplasms / Q000201:enzymology*; D054306:Dideoxynucleotides; D005260:Female; D006367:HeLa Cells; D019337:Hematologic Neoplasms / Q000201:enzymology*; D006801:Humans; D007124:Immunoenzyme Techniques / Q000379:methods*; D000091402:Prohibitins; D012044:Regression Analysis; D015203:Reproducibility of Results; D013937:Thymidine Kinase / Q000097:blood*; D013942:Thymine Nucleotides / Q000378:metabolism*; D015215:Zidovudine / Q000031:analogs & derivatives* / Q000378:metabolism*
D014408:Biomarkers, Tumor; D054306:Dideoxynucleotides; C000717331:PHB2 protein, human; D000091402:Prohibitins; D013942:Thymine Nucleotides; C057287:3'-azido-3'-deoxythymidine 5'phosphate; D015215:Zidovudine; D013937:Thymidine Kinase; C092717:thymidine kinase 1