1. Isoquinoline, cinnoline, quinoxaline, quinazoline and phthalazine were incubated with preparations of rabbit liver aldehyde oxidase. 2. The oxidation products, 1-hydroxyisoquinoline, 4-hydroxycinnoline, 2-hydroxy- and 2,3-dihydroxy-quinoxaline, 4-hydroxy- and 2,4-dihydroxy-quinazoline, and 1-hydroxyphthalazine were identified by comparison of their spectral and chromatographic characteristics with those of authentic compounds. 3. Michaelis-Menten constants are reported for the action of the parent heterocycles with aldehyde oxidase. The compounds reported in this study are among the most efficient substrates yet described for rabbit liver aldehyde oxidase. 4. The compounds in 1 above were incubated with bovine milk xanthine oxidase: only quinazoline and phthalazine yielded significant amounts of metabolites. Km values were calculated for these compounds. 5. Incubation of the heterocycles with rat liver preparations gave qualitatively the same results as those obtained using rabbit liver, but smaller amounts of the oxidation products were detected from rat liver incubations.

Quinolinium salts were incubated with partially purified aldehyde oxidase, and the products were separated by high-pressure liquid chromatography and fully characterized by u.v. spectroscopy, i.r. spectroscopy and mass spectrometry. Oxidation of N-methylquinolinium salts with either rabbit or guinea-pig liver aldehyde oxidase in vitro gave two isomeric products, N-methyl-4-quinolone and N-methyl-2-quinolone. Incubation of N-phenylquinolinium perchlorate similarly yielded two oxidation products, N-phenyl-4-quinolone and N-phenyl-2-quinolone. The ratio of 2- to 4-quinolone production was species-dependent, the proportion of 4-quinolone with the guinea-pig enzyme being greater than that obtained with the rabbit liver enzyme. Kinetic constants were determined spectrophotometrically for both the quinolinium salts and a number of related quaternary compounds. In general, quaternization facilitated oxidation of a substrate, but a number of exceptions were noted, e.g. N-methylisoquinolinium and N-methylphen-anthridinium. Km values varied with the nature of electron acceptor employed, and this difference was more marked for quaternary substrates than the unquaternized counterparts. The product ratio obtained from N-methylquinolinium salts was found to be constant under various conditions, including purification of the enzyme and the use of either induced or inhibited aldehyde oxidase, but a change in the ratio was found at high pH values and in the presence of a competing substrate, N-methylphenanthridinium. This may indicate that a quaternary substrate binds to aldehyde oxidase in two alternative positions.

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DNA showed a 20 per cent decrease in the incorporation of labelled UMP while under the conditions for the RNAprimed reaction there was a slight increase in incorporation.No apparent differences were observed in the activities of DNA nucleotidyltransferase or DNA-primed RNA nucleotidyltransferase in preparations from infected and uninfected cells and cell fractions (Fig. 1a, b). However, significant increases in RNA-primed RNA nucleotidyltransferase activity were observed in each of the five experiments of this type performed beginning at intervals of 1-3 h after infection (Fig. 1c). The increases were more pronounced in enzyme preparations from nuclear than cytoplasmic fractions.Reich, Franklin, Shatkin and Tatum• have demonstrated that more than 90 per cent of DNA-primed RNA synthesis can be inhibited by actinomycin D in L cells without significant interference with the production of an infectious RNA virus. More recently, Baltimore and Franklin 7 have found that after infection of L cells with mengovirus there is increased incorporation of ribonucleotide residues into polyribonucleotides by a particulate fraction from the cell cytoplasm. These pieces of evidence, together with the finding that RNA-primed RNA nucleotidyltransferase activity is increasod in Krebs II ascites tumour cells on infection with encophalomyocarditis virus, suggest that the rtplication of RNA viruses, at least in some mammalian systems, is dependent on RNA-primed rather than on DNA-primed synthesis of RNA.