The role of uridine-cytidine kinase in the regulation of pyrimidine ribonucleotide synthesis in Tetrahymena pyriformis GL

Purification and characterization of two vascular endothelium effectors from fetal bovine retina, vitreous and serum

The purification and characterization of uridine and thymidine, two previously isolated vascular endothelium effectors, are described. These two effectors were purified from all dialyzable fractions of fetal bovine serum (FBS), vitreous, and retina. The present study demonstrates the stimulatory characteristic of uridine and thymidine on the proliferation of aortic endothelial cells. The stimulation appears to link to their utilization by the cells in a dialyzed FBS-dependent manner. In the presence of optimal concentrations of uridine (10 microM), thymidine (0.8 microM), and dialyzed FBS (3 mg ml-1), a markedly enhanced vascular endothelial cell proliferation is attainable; uridine and thymidine alone had no significant stimulatory effect. The presence of both nucleosides, however, increased the concentration of dialyzed FBS required for half-maximum proliferation activity of fetal bovine aortic endothelial cells and also increased maximum activity. Studies with fetal bovine aortic endothelial cells showed that the activity of uridine kinase in the cells was activated by dialyzed FBS, but not by uridine and thymidine. Uridine and thymidine exerted no marked stimulatory effect on Balb/c 3T3, Swiss 3T3, and SV-T2 as noted with endothelial cells. The significance of the present findings to the proliferation of vascular endothelial cells in the retina is discussed.

Preliminary studies of the uridine kinase activity of human colorectal adenocarcinomas

Relatively low uridine-cytidine kinase activities in 22 surgical specimens of human colorectal adenocarcinomas have been demonstrated, in comparison with those of other neoplasms studied. The kinase activities averaged 1.1 nmoles/min/mg of protein; however, four of the specimens exhibited significantly higher levels of enzyme activity than the average, a deviation that could not be correlated with any properties of the tumors. Of the 22 neoplasms, 86% were moderately differentiated; and 14% were poorly differentiated. Staging disclosed 9% Dukes' A neoplasms, 41% Dukes' B, and 50% Dukes' C. The moderately differentiated group included four mucinous tumors; the poorly differentiated only one; none was of the signet-ring cell type. The kinetic characteristics of the uridine kinase activity of one colonic adenocarcinoma were studied in detail; the apparent Michaelis constants for uridine and adenosine 5'-triphosphate were 0.37 x 10(-3) M and 1.07 x 10(-3) M, respectively, and the enzyme required Mg2+ for maximal activity (at 30-42 C).

Non-allosteric regulation of the uridine kinase from seeds of Zea mays

Uridine kinase (ATP: uridine 5'-phosphotransferase, EC 2.7.1.48) has been partially purified from ungerminated hybrid corn seed. It is associated with a soluble high molecular weight fraction from which it apparently cannot be dissociated without loss of activity. The stability of the enzyme is enhanced by the addition of dithiothreitol, glycerol and nucleotide substrate. The nucleoside specificity of the enzyme is limited to nucleosides containing pyrimidine and ribose moieties, such as uridine and cytidine. High concentrations of nucleosides cause substrate inhibition, however. The Km values for uridine and cytidine are 53 muM and 125 muM, respectively, and under subsaturating conditions uridine is phosphorylated about five times faster than cytidine. The reaction follows an ordered Bi Bi kinetic pattern, with ATP and ADP in competition for the free form of the enzyme. Purine, but not pyrimidine, nucleoside triphosphates serve as phosphate donors without regard to the sugar moiety. However, all of these triphosphates appear to compete for the same site on the enzyme. (Km ATP equals 590 muM, Km (app) GTP equals 61 muM, and CTP and UTP are linear competitive inhibitors against ATP, with Ki values of 60 muM and 240 muM, respectively.) Therefore, end product control of uridine kinase apparently does not involve allosteric sites, but instead is envisioned as simple competition between relatively effective or ineffective phosphate donors for a position on the enzyme.