Conformational changes in transfer RNA. I. Equilibrium theory

We report the results of semi-empirical calculations describing thermodynamic properties of transfer RNA conformations. The most important new features of the procedure are: (1) the use of parameters obtained from model oligoribonucleotides to evaluate the free energy of helical regions and small hairpin loops, and (2) the use of a model which is somewhat more realistic than the freely jointed chain for evaluating internal loops and intermediate size hairpin loops. The new parameters lead to important quantitative and qualitative differences from predictions which would have been made in the past and lead to a priori predictions of tRNA melting temperatures which are within about 6°C of the experimental values. The results suggest the following conclusions: (1) The early melting transition observed in several tRNA's is partly the result of tertiary unfolding, and partly the result of the loss of some secondary structure. (2) The part of the secondary structure which melts during the early transition is different for different tRNA's. For fMet and Tyr from E. coli, the calculations predict that the dihydrouradiene arm melts out early. For yeast Phe the acceptor stem and anticodon helix melt first. (3) The results also suggest the possibility that tertiary unfolding and early secondary structural melting do not occur independently but are coupled, so that the two types of structure are probably mutually stabilizing.     

Oxidative metabolism of carbon disulphide by the rat. Effect of treatments which modify the liver toxicity of carbon disulphide

The liver toxicity of a standard dose of CS₂ was increased by phenobarbitone pre-treatment and by fasting and decreased by prior administration of a small dose of CS₂; it was less marked in female than in male rats.Rats were given an intraperitoneal dose of ¹⁴CS₂ to study its in vivo conversion to ¹⁴CO₂ under conditions which modify the liver toxicity of CS₂. The radioactive CO₂ exhaled was found to be linearly related to the amount of cytochrome P-450 present in the liver at the time of administering ¹⁴CS₂; it was also found to be related to the severity of the toxic changes caused in the liver, but here the relationship was not linear and was more complex.When liver microsomes were incubated with CS₂, a marked loss of cytochrome P-450 was observed only when NADPH was present. In contrast with the loss of the cytochrome observed with NADPH alone, which is associated with the formation of malonaldehyde, that due to CS₂ still occurred in the presence of EDTA, even though the formation of malonaldehyde was largely prevented.These results are compatible with the hypothesis that CS₂ requires metabolism for its liver toxicity.