The activity of
uridine kinase (
ATP:
uridine 5'-
phosphotransferase; EC 2.7.1.48), the rate-limiting
enzyme of the
UMP salvage pathway, was measured in human ovaries and ovarian
carcinomas, in a spectrum of six rat
hepatomas of different growth rates and in eleven normal rat tissues of high and low cell renewal rates. In a standard isotopic method developed for the 100,000 x g fraction,
uridine kinase activity was linear for 20 min and proportional with
protein concentration over a range of 0.1 to 0.8 mg per 0.1 ml reaction mixture. The apparent Kms for
uridine,
ATP and Mg++ in normal rat liver were 5.0, 3.4 and 1.5 mM and in the rapidly growing
hepatoma 3924A, 0.8, 2.1 and 1.1 mM, respectively. In normal control ACl/N and Buffalo strain rat livers,
kinase activity ranged from 159 to 180 nmol/h/mg
protein. In
hepatomas of slow and intermediate growth rates,
kinase activity increased to 1.5- to 2.6-fold, and in
hepatomas of rapid growth rates, to 5.1- to 5.8-fold over that of the relevant control, normal livers. When
hepatoma 3924A tissue culture cells were plated and expressed their proliferative program,
kinase activity increased to 2.1-fold in early log phase. To further clarify the linkage between
uridine kinase and cell replicating capacity, the
enzyme activity was measured in rat organs of high and low cell renewal. The
kinase activity in liver of adult male Wistar rats was 176 +/- 6 nmol/h/mg
protein. Activities in thymus, spleen and bone marrow were 4.7-, 2.1-, and 1.8-fold, respectively, of rat liver values; in adipose tissue, the activities were low. The decay rates of
uridine kinase were examined in rats injected with a high dose of
cycloheximide, which inhibits protein biosynthesis by 90%. The t(1/2) of the
kinase in rat bone marrow was 0.64 h, in rat liver longer than 6 h. In human ovary and ovarian
carcinoma, the apparent Kms for
uridine were 11.5 and 0.5 mM, respectively. In human ovary (n = 3),
kinase activity was 38 nmol/hr/mg
protein; in ovarian
carcinoma (n = 6), the activity increased to 5- to 13-fold over that in ovary. The positive linkage of
uridine kinase activity with proliferation and transformation is apparent in human ovarian
carcinomas and in rat
hepatomas of different growth rates. Therefore, the increased
uridine kinase activity should be an interesting target for anticancer
chemotherapy.