Regulation of the level of S-adenosylmethionine decarboxylase during lymphocyte mitogenesis.

Bovine lymphocytes were mitogenically activated by the lectin concanavalin A and the activity of the polyamine biosynthetic enzyme, S-adenosylmethionine decarboxylase (SDC), was followed (R. H. Fillingame and D. R. Morris, Biochem. Biophys. Res. Commun. 52: 1020-1025, 1973). Enzyme activity began to increase at approximately 3 hours after cellular activation, reaching a peak at 9-12 hours. A second elevation of SDC activity coincided with the entry of the cells into S phase at 24 hours. The early and late increases in SDC activity were regulated by different mechanisms. The initial elevation of activity was due to approximately a 10-fold enhancement of the rate of enzyme synthesis; the enhanced rate of synthesis was maintained throughout the remainder of the culture period. The second increase in SDC activity was due to a lengthening of the intracellular half-life of the enzyme from 80 to 170 min. These changes in the rates of SDC synthesis and degradation fully account for the observed biphasic enzyme induction curve. In vitro translation of the SDC messenger RNA has been achieved and this will be used in further studies of the mechanism of enhanced enzyme synthesis.