[Thymidine kinase in normal and leukemic myeloid cells]

Acute and temporary inhibition of thymidine kinase in mouse bone marrow cells after low-dose exposure

Low-dose whole-body exposure of mice to less than 0.01 Gy gamma-rays causes inhibition of incorporation of thymidine or 5-iodo-2-deoxyuridine into DNA of bone-marrow cells in vitro; the effect is maximal in cells at 4 hours after exposure and then subsides within about 10 hours. This is due to the inhibition of cellular thymidine kinase, which gradually develops to a maximum at 4 hours after exposure and again subsides within the next 10 hours. This inhibition involves only 35 per cent of the entire cellular enzyme activity and, analogous to the depression of thymidine incorporation into the cells, is only seen when the cells are collected into medium that is buffered to 7.2-7.4 and contains about 1350 mg NaHCO3 per litre. Addition of NaHCO3 to the cell homogenate or to the high speed supernatant containing the enzyme, but not to intact cells, failed to produce enzyme inhibition. There is also no depression of 5-iodo-2-deoxyuridine uptake into the intact cells in vitro when the mice are irradiated either shortly before or after i.v. injection of 0.02 mg of procain chloride. The reversibility of the effect in vivo and in vitro suggests a particular enzyme control mechanism that may be non-specifically triggered by intracellular charges, such as peroxides, and may enhance repair.

Disassembly of microtubules in the Lesch-Nyhan Syndrome? (Lesch-Nyhan syndrome and microtubules)

The Lesch-Nyhan syndrome is an unusual disease. It combines neurological disorders, behavioural disturbances, metabolic changes and haematological symptoms. The syndrome is caused by an X-chromosomal transmitted enzyme deficiency of the 'salvage pathway' in purine metabolism. The hitherto unexplained pathogenesis was the reason for investigations into metabolism and morphology of the blood cells of a patient suffering from the syndrome. Along with the defect in guanine nucleotide resynthesis there was a defect of microtubules in platelets and a sphaerocytosis in red cells, which could be the result of a disassembly of structural proteins. The development and maintenance of the highly heteromorphic structure of nerve cells and the neuronal function including axonal transport of cell organelles and transmitters is dependent on microtubules. Thus a disassembly of microtubules could be the mechanism in the pathogenesis of this complex syndrome.

Further in vitro studies on the biochemistry of the inhibition of nucleic acid and protein synthesis induced by arsenic

Human lymphocytes, stimulated by PHA, and exposed to increasing Na2HAsO4 concentrations, show an identical incorporation rate for 14C thymidine and 14C-TTP into the DNA. 14C uridine is incorporated 3--4 per cent less at an As concentration of 1.0 mug/ml medium, above this however approximately 15 per cent less into the RNA as compared to 14C-UTP. In free pyrimidine bases, the incorporation of labelled triphosphates into the DNA and RNA is significantly reduced above 1 mug and 10 mug Na2HAsO4. Corresponding to its approximately uniform distribution into DNA and RNA, the incorporation rate of 14C-ATP above 10 mug Na2HAsO4/ml culture medium lies between that of 14C-UTP and 14C-TTP. The incorporation of 14C alanine and 14C leucine into cellular protein is not reduced below 10 mug Na2HAsO4/ml. The incorporation rate is 41 per cent at a concentration of 100 mug Na2HAsO4/ml medium. Compared to 14C-UTP, 14C-ATP and 14C-TTP it is increased by the factor of 1.8; 2.5 and 6.8 respectively. The inhibition of enzymes of the dark repair mechanisms and the synthesis of biopolymers together with their altered sequence and the involvement of long-lived messenger RNA serve as an explanation of the observed alterations of the lymphocyte metabolism, caused by arsenic.