By the method of gel electrophoresis, radiation-induced DNA single- and double-strand breaks (SSB, DSB) were studied with a model system of pBR322 solution in vitro in the presence of •OH radical scavengers, mannitol and TE (10-2 mol dm-3 Tris and 10-3 mol dm-3 ethylene diamine tetra-acetic acid). Experiments showed that SSB resulted from one-hit events of radiation energy deposition and DSB resulted from both one-hit and two-hit energy deposition events and so were distinguished into two classes of αDSB and βDSB. Moreover, α/β, where a is the number of DSB per unit dose induced in one irradiation event and β the number of DSB per unit squared dose induced by the combination of two independent SSB, was related to the scavenging capacity, s, and for σ<108 s-1, αDSB predominate over DSB. On the other hand, if σ<2´108 s–1, the measured G(αDSB) decreased in parallel with G(SSB), i.e., G(αDSB)/G(SSB) was a constant. When σ>2×108 s-1, G(αDSB) decreased slightly so that the ratio of αDSB to SSB evidently increased. Therefore, αDSB could be induced by the radical transfer mechanism for σ<2×108 s-1 and contrarily produced by the local multiply damaged sites (LMDS) mechanism for σ larger than this value. In addition, the distance for two independent complementary SSB forming αDSB was deduced, but no apparent variation of it was found in the wide σ range from ～105 to ～109 s-1, which shows that the DNA steric structure was not influenced by mannitol.

Discharge was performed against ammonia water using a graphite rod as the anode and a silver thread as the cathode under an Ar atmosphere. HPLC and thin layer chromatograph (TLC) analyses showed that three kinds of amino acids were produced in the reaction mixture. As the graphite anode is the solitary source of carbon in the system, it is considered that amino acids have been produced by synthetic reaction between graphite and ammonia water. Thus, our results provide a possible way of formation of amino acids from elemental carbon on the primitive earth. In ad-dition, the mechanism for the production of amino acids is discussed and the yields of different amino acids are pre- sented.

With a model system of pBR322 plasmid DNA solution in vitro, the dose effects of radiationinduced single- and double-strand breaks (SSB and DSB) were measured and DSB was distinguished into a- and b-types. Under the condition of low scavenging capacity existing in the irradiated DNA solution, SSB and aDSB were mainly induced by hydroxyl radicals (•OH). Moreover, a certain relationship was obtained between the SSB and aDSB yields and the DNA concentration. It was found that when the DNA solution was irradiated in the presence of 2.5 mmol dm-3 mannitol, the reciprocals of G (SSB) and G (aDSB), respectively, were linearly related to the reciprocal of the DNA concentration, i.e. the competition reactions of DNA and mannitol for •OH radicals can be described by second-order kinetics. The rate coefficients and the efficiencies of the •OH radical inducing SSB were deduced. Also, the reaction rate coefficients and the efficiencies for the induction of aDSB from SSB by the •OH radical transfer mechanism, were first derived from the competition kinetics.

i per nuclear traversal, sug-gesting the existence of bystander effects. This low-dose sen-sitivity was increased when GJIC was enhanced by treating cells with 8-Br-cAMP, but it was partly reduced by treating cells with DMSO, an effective scavenger of reactive oxygen species (ROS). Moreover, no low-dose sensitivity was observed when cells were treated with 100 μM lindane, an inhibitor of GJIC. The survival of irradiated cells was increased by DMSO but was not influenced significantly by cAMP or lin- dane. On the other hand, G1-phase arrest was detected in the irradiated cells, and it was enhanced by cAMP. In contrast, this arrest was reduced or almost eliminated by DMSO or iindane, respectively, even when cells were irradiated with such a high dose that each cell received five nuclear traversals on average. Thus the bystander responses occurred after both low-dose and relatively high-dose irradiation. Our results in-dicated that both GJIC and ROS contributed to the radiation- induced bystander effect, but gap junctional channels might play an essential role by modulating the release of radiation-induced signaling factors.

Nitric oxide (NO) is an important messenger molecule with multiple biological activities. In the present study, sper/NO, a NO generator, showed a biphasic effect on the proliferation of human salivary gland neoplastic (HSG) cells. Sper/NO of less than 20 lM stimulated cells to depart from the G2/M phase and so enhanced cell division and cell proliferation. But sper/NO at higher concentrations restrained cell proliferation and blocked cell-cycle progression. Cells were mainly arrested in the G2/M phase and S phase when they were treated with 100-200 and 300-500 lM sper/NO, respectively. Aspecial S-phase peak was detected in a histogram of the cell-phase distribution of sper/NO-treated HSG. When the concentration of sper/NO increased, the S-phase peak shifted from early the G2/M-phase to later the G1–S-phase boundary. Sper/NO-induced cell-cycle arrests were reversible when the cells were released from NO stress for 48 h and hence cell proliferation was recovered. In addition, micronucleus, but no apoptosis, was produced in the sper/NO-treated cells, and its yield tended to a saturation value with increasing concentrations of sper/NO. The sper/NO-induced effects were effectively eliminated or reduced by treating cells with PTIO, a NO-specific scavenger, indicating that NO is the main source of these effects.