Characteristics of the carcinogenic effectiveness of 645 MeV protons and 60Co gamma-irradiation

The carcinogenic effect of 645 MeV protons and 60Co-gamma-rays (1, 2 and 4 Gy) on mongrel female rats was comparatively studied. The observations were made throughout the entire life span of the exposed animals. Revealed were a similar biological effectiveness of the two types of radiation and a common nature of the carcinogenic transformations.     

Effect of methylamine on the reaction of alpha 2-macroglobulin with enzymes.

The kinetics of reaction of alpha 2-macroglobulin (alpha 2M) with thrombin and with trypsin were studied in the presence and absence of methylamine. The rate of enzyme-induced thiol release was found to be the same whether or not amine was present. The result suggests that covalent bond formation and enzyme-catalyzed amine incorporation proceed via a common (enzyme-dependent) rate-determining step. The reaction of lysyl-modified enzymes (which show poor covalent binding with alpha 2M) was similarly unaffected by amine, indicating that enzyme-catalyzed steps were also rate determining for hydrolysis of the thiol ester. The products of the reactions were analyzed by native and denaturing gel electrophoresis. Methylamine did not affect the total binding of enzyme to alpha 2M but did cause a substantial decrease in covalent binding. Surprisingly, not all covalent complexes were affected by the presence of amine: complexes in which enzyme was covalently bound to one half-molecule increased compared to the reaction with no amine; complexes in which two half-molecules are cross-linked by two bonds to a single enzyme were substantially reduced, however. The results are consistent with a mechanism of reaction in which an enzyme-dependent step is rate determining. This step is accompanied by activation of two thiol esters. One of these reacts immediately with the bound enzyme (or may be hydrolyzed if the enzyme amine groups are blocked). The other activated center is capable of reaction with external nucleophiles such as methylamine.     

Effects of 60Co gamma-irradiation of mice on the temporal changes of acid phosphatase activity in spleen and liver.

Acid phosphatase activity and protein content of spleen and liver, and organ weight of whole-body 10 Gy 60Co gamma-irradiated mice were measured every four hours during a 24-hour period. In irradiated mice, in comparison with those non-irradiated, increased acid phosphatase activity in spleen related to both 1 mg of protein at 20.00I, 04.00, 08.00, 12.00, 16.00 and 20.00II and 1 g of fresh tissue at 20.00I, 08.00, 12.00, 16.00 and 20.00II; decreased weight of spleen and protein amount in spleen during the whole 24-hour period, as well as fluctuations in all the parameters measured in spleen, except the level of protein related to 1 g of fresh tissue, were observed. In irradiated mice, compared with the controls, the increased acid phosphatase activity in liver calculated per both 1 mg of protein at 24.00, 08.00 and 16.00 and 1 g of fresh tissue at 08.00 and 16.00; the decreased protein concentration in liver related to 1 g of fresh tissue and the whole organ weight at 12.00, as well as temporal changes in the protein level in liver expressed per 1 g of fresh tissue, were found. 60Co irradiation of mice influenced the acid phosphatase activity and protein concentration in liver are less than in spleen.     

Effect of gamma-irradiation on dye-DNA binding.

The effect of gamma-rays on the binding of proflavine and acridine orange to DNA was investigated by spectrophotometry. The effect of irradiation was observed on the buffered solutions of the free dye and free DNA. A dose of about 35 krad caused a hyperchromicity of 30-40 per cent to the DNA peak at 258 nm, while the same dose introduced a hypochromic effect to the monomer peaks of the dyes by 30 per cent. This implied that gamma-rays have an effect of decreasing the monomer concentration of free-day molecules in solution. From the results, we conclude that more dye is bound to the changed conformation of dye-bound DNA on irradiation. Scratchard-binding isotherms drawn for the unirradiated and irradiated complexes of Pf-DNA showed interesting differences. Similar isotherms could not be obtained for the acridine orange-DNA system.     

Cytogenetic effect of low-level 60Co gamma-irradiation of cultured human lymphocytes in the G1 phase of the mitotic cycle

The dose-dependence of the chromosome aberration frequency in human lymphocytes in vitro exposed to 60Co-gamma-radiation at the G1 stage of the mitotic cycle has proved to be unlike that obtained upon exposure of cells at the G0 stage of the cycle. The data obtained are accounted for by partial activation of the repair system at the G1 stage which is attributed mainly to chromatin decondensation.     

Effect of gamma-irradiation on oocysts of Eimeria necatrix.

Effect of gamma radiation on oocysts of Eimeria necatrix was investigated. It was observed that oocysts exposed to 200 kR or above did not sporulate. Irratiation at 10-150 kR caused a progressive decrease in sporulation. Irradiation affected normal development of unsporulated oocysts as the zygote protoplasm divided into unequal masses or was shattered into granules. Increase in the intensity of irradiation of sporulated oocysts resulted in the progressive decrease in severity of the resultant infections in chicks and their effects - mortality, type of lesions developed, total oocyst production and immunity produced - were comparable with infections induced by decreasing the number of unirradiated oocysts. Infection produced by 1000 unirradiated oocysts was comparable with that resulting from 50 000 oocysts irradiated at 25 kR. Infection obtained with 20 000 unexposed oocysts approximated to that produced by 50 000 oocysts irradiated at 2-5 kR. It was concluded that irradiation abolished infectivity of the oocysts/sporozoites rather than bringing about attenuation of the parasite.     

Excision of apurinic sites from DNA with enzymes isolated from rat-liver chromatin

Apurinic sites were excised from phi X174 RF DNA with two enzymes isolated from rat liver chromatin: an apurinic/apyrimidinic endodeoxyribonuclease and a 5'-3'-exonuclease; the resulting gap was filled with DNA polymerase beta also prepared from rat liver chromatin and the repair was fully terminated with T4 ligase.     

The internal Cys-207 of sorghum leaf NADP-malate dehydrogenase can form mixed disulphides with thioredoxin

The role of the internal Cys-207 of sorghum NADP-malate dehydrogenase (NADP-MDH) in the activation of the enzyme has been investigated through the examination of the ability of this residue to form mixed disulphides with thioredoxin mutated at either of its two active-site cysteines. The h-type Chlamydomonas thioredoxin was used, because it has no additional cysteines in the primary sequence besides the active-site cysteines. Both thioredoxin mutants proved equally efficient in forming mixed disulphides with an NADP-MDH devoid of its N-terminal bridge either by truncation, or by mutation of its N-terminal cysteines. They were poorly efficient with the more compact WT oxidised NADP-MDH. Upon mutation of Cys-207, no mixed disulphide could be formed, showing that this cysteine is the only one, among the four internal cysteines, which can form mixed disulphides with thioredoxin. These experiments confirm that the opening of the N-terminal disulphide loosens the interaction between subunits, making Cys-207, located at the dimer contact area, more accessible.