Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.2.1.23 (beta-galactosidase)
 14,648 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Thymidine kinase (TK) is a nucleotide salvage pathway enzyme whose activity is highly dependent on the growth state and cell cycle phase of a cell. Cells in the resting or quiescent (G0) phase express very low levels of TK mRNA and protein. When quiescent cells are stimulated to enter the cell cycle by the addition of serum, TK mRNA, activity and polypeptide increase coordinately after about 10-15 h, at the beginning of S phase. When growth-independent heterologous promoters are substituted for the natural TK promoter, TK mRNA can be expressed in quiescent cells. Despite the presence of TK mRNA in such G0 cells, there is little expression of TK polypeptide; the normal increase in enzyme at S phase is observed following serum stimulation. Deletion of the introns and 3' untranslated sequences does not affect the expression of the TK gene in serum stimulation experiments. In contrast, deletion of the C-terminal 40 amino acids or fusion of a small segment of a beta-galactosidase to the C-terminus overcomes the block to expression of the TK polypeptide in G0 cells. These C-terminal alterations are the same as those which lead to constitutive expression of TK during the cell cycle of proliferating cells, suggesting that mechanisms which control the levels of TK in cycling cells may also operate in quiescent cells.
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PMID:Mutations in the thymidine kinase gene that allow expression of the enzyme in quiescent (G0) cells. 190 30

Analyses of enzymes from various metabolic pathways in pulmonary carcinoid tumors and radiological measurements of their volume increase were compared with those for lung carcinomas of various cell types. The results describe new biochemical features in carcinoid tumors, present the first quantitative evidence for their slow growth rate (i.e., long doubling time) in vivo, and show that measurement of 2 or 3 appropriate enzymes in biopsy samples can guard against instances in which carcinoids and adeno- or oat cell carcinomas are mistaken for one another on histological examination. The uridine kinase to thymidine kinase ratio as well as the beta-galactosidase concentration of carcinoid tumors were 5 times higher than of carcinomas, and their gamma-glutamyl transpeptidase was below that of all 35 adeno- and the 11 squamous cell carcinomas. Thymidine kinase, which bears a quantitative inverse correlation to volume doubling time (irrespective of cell type), had much lower titers in the 9 carcinoids than in the 6 oat cell carcinomas and reflects most clearly their very different (despite common histogenesis) clinical malignancy. Owing to their long doubling time, carcinoid tumors on the average required a much longer period (40.5 years) to attain final volume than did carcinomas (17.8 years). The calculated mean age of the subjects when growth began, -0.5 years (as opposed to 51 years for carcinomas), suggests a prenatal or early childhood inception for pulmonary carcinoid tumors.
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PMID:Pulmonary carcinoid tumors: enzymic discriminants, growth rate, and early age of inception. 287 Jul 99

Replication-dependent changes in levels of enzymes involved in DNA precursor biosynthesis are accompanied frequently by changes in levels of cognate mRNA. We tested the common assumption that changes in mRNA levels are responsible for growth-dependent expression of these enzymes using a line of mouse muscle cells that irreversibly withdraws from the cell cycle as part of its terminal differentiation program. Thymidine kinase (TK) mRNA, activity, and protein levels were quantitated in cells transformed with multiple copies of the chicken TK gene. The decline in TK mRNA (both whole cell and cytoplasmic) during myogenesis was poor (2-fold average) and variable (1.2 to 8-fold). In contrast, TK activity always was regulated efficiently (20-fold), even in cells which regulated TK mRNA very poorly. Thus, regulation of TK activity was independent of TK mRNA regulation as myoblasts withdrew from the cell cycle. A TK/beta-galactosidase fusion protein was used to derive an antibody against chicken TK. Immunoblot and immunoprecipitation analyses demonstrated TK protein levels, like TK activity levels, declined to a greater extent than TK mRNA levels. Thus, TK activity likely was regulated by a mechanism involving either decreased translation of TK mRNA or increased degradation of TK protein in committed muscle cells.
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PMID:Regulation of thymidine kinase protein levels during myogenic withdrawal from the cell cycle is independent of mRNA regulation. 321 45