Determination of serum
thymidine kinase 1 (STK1) activity has been used as a proliferation marker for neoplastic diseases in both human and veterinary medicine. The purpose of this study was to determine STK1 activity and
enzyme levels in different dog tumours. Serum samples from three dogs with leukaemia, five with
lymphoma, 21 with solid tumours and 18 healthy dogs were analyzed for STK1 activity, using an optimized [(3)H]-
deoxythymidine (dThd) phosphorylation assay, and for STK1
protein levels using an immunoaffinity/western blot assay. STK1 activity in dogs with haematological tumours was significantly higher than in the solid tumour and healthy dog groups (mean ± standard deviation [SD] = 65 ± 79, 1.1 ± 0.5, and 1.0 ± 0.4 pmol/min/mL, respectively). Serum samples were analyzed after immunoaffinity isolation by western blot and the TK1 26 kDa band intensities quantified revealing that concentrations were significantly higher in dogs with haematological tumours and solid tumours compared to healthy dogs (mean ± SD=33 ± 12, 30 ± 13, and 10 ± 5 ng/mL, respectively). Pre-incubation with the
reducing agent dithioerythritol (DTE) showed a decrease in STK1 activity and
protein levels in most samples, but an increase of about 20% in sera from healthy dogs and from those with haematological
malignancies. Compared to animals with solid tumours, the specific STK1 activity (nmol [(3)H]-
deoxythymidine monophosphate (
dTMP)/min/mg of TK1
protein of 26 kDa) was 30-fold higher in haematological
malignancies and 2.5-fold higher in healthy dogs, respectively. The results demonstrate that there is a large fraction of inactive TK1
protein, particularly in sera from dogs with solid tumours. The findings are important in the use of STK1 as a
biomarker.