RNA sulfurtransferase activity will be measured in both normal liver and chemically induced hepatomas. This is made possible through the use of sulfur-deficient tRNA from the C6 mutant of Escherichia coli as substrate for the in vitro assay of these enzymes. The requirements of the system will be defined and the rate of the reaction will be measured in the presence of various concentrations of the required components. Control experiments will be carried out with enzyme preparations from fetal, neonatal, and regenerating livers, to determine whether sulfertransferase levels can be correlated with differentiation and growth. Using the heterologous substrate, the base specificity of the liver and hepatoma sulfurtransferases will be investigated. The tRNA reaction product will be digested and the thionucleotide content determined by paper and column chromatography. Finally, perfused liver will be used to elucidate the sulfur donor in thionucleotide synthesis, and to prepare radioactively labeled liver tRNA. The latter will be digested and the thionucleotide content measured as described above. The results of these studies will be compared to those obtained with the tRNA methylases of liver and hepatoma tissue, to determine whether other tRNA modification enzymes are also elevated in cancer tissues. In addition to defining a new enzyme system in mammalian cells, this investigation will provide further insight as to the significance of tRNA hypermodification in tumors.