Promotion by SF6 of a Radiolytically Induced Chain Reaction for the Geometrical Isomerization of cis or trans-Butene-2 in Dilute Gaseous Mixtures
FOR several years SF6 has been used in radiolytic studies because of its high cross-section for capture of low-energy electrons and consequent effect on observable products resulting from positive-ion neutralization reactions. A reduced yield of H2 from irradiated propane has been reported1 and attributed to electron scavenging by the small proportions of SF6 present. On the other hand, SF6 does not affect the H2 yield from irradiated liquid unsaturates such as trans-butene-2 (ref. 2), indicating that ionic species may not be important with respect to H2 formation from these olefines.
SF6 added to irradiated gaseous ethylene acts not only as an electron scavenger
Abstract The reduction in the yields of C 4 unsaturated hydrocarbons from C 2 H 4 NH 3 mixtures in the presence of SF 6 is due to secondary processes involving a radical derived from SF 6 .
Cytoplsmic uridine phosphorylase of rat liver. Characterization and kinetics.
Abstract Uridine phosphorylase (uridine:orthophosphate ribosyltransferase, EC 2.4.2.3) of rat liver cytoplasm has been purified 1900-fold. Initial velocity patterns of the phosphorolysis of uridine or deoxyuridine and of the synthesis of uridine indicated a sequential mechanism. Product inhibition patterns with uracil or ribose 1-phosphate as inhibitors were consistent with an "ordered Bi Bi" reaction in which inorganic phosphate is the first substrate to bind to the enzyme and ribose 1-phosphate the last product to leave the enzyme. The most highly purified enzyme fractions still retained uridine-, deoxyuridine-, and thymidine-cleaving activities in the ratio of 10:7:1 at pH 7.4. The fractions separated into a major and a minor band of protein on electrophoresis in phosphate buffer. Aging of the enzyme with loss of activity resulted in an increase in the proportion of the minor band. The pH optimum of the enzyme for uridine cleavage was 8.2, for deoxyuridine cleavage, 6.5, and for uridine synthesis, 8.5. Uridine or deoxyuridine synthesis was inhibited 50% in the presence of 1 mm uracil. The enzyme did not catalyze the direct transfer of ribose from uridine to uracil in the absence of phosphate. Uridine protected the enzyme to a greater degree than phosphate against inhibition by o-iodosobenzoate. Thus, at least one sulfhydryl group and possibly three are present at or near the active site. Uridine cleavage, uridine synthesis, and deoxyuridine cleavage were inhibited by deoxyglucosylthymine. The phosphorolysis of deoxyuridine and thymidine by thymidine phosphorylase, also purified from rat liver cytoplasm, was not inhibited by deoxyglucosylthymine. The molecular weights of both uridine and thymidine phosphorylases were estimated to be 110,000.