Ornithine decarboxylase and ribosomal RNA synthesis during the stimulation of lymphocytes by phytohaemagglutinin

Abstract

Omithine decarboxylase (ODC) catalyses the first step in the biosynthesis of polyarnines, the conversion of omithine to putrescine [l] . Its activity rises dramatically at an early stage in the conversion of many types of cells from a non-growing to a growing state [2-41 and it has also been reported to have an unusually short half life [5] . The consequent rapid fluctuations in enzyme activity in response to growth stimulation suggest that this enzyme might be involved in cell growth regulation mechanisms. Prominent among the sequence of metabolic changes invariably associated with the change from the non-growing to the growing state is an early increase in the synthesis of ribosomal RNA (rRNA), and there are some suggestions that ODC activity may be associated with this [6,7] . The effect of growth stimulation on rRNA synthesis has been studied in most detail during the activation of cultured human lymphocytes by phytohaemagglutinin (PHA). Separate effects of the growth stimulant on the synthesis of ribosomal precursor RNA and on the rate and efficiency of the maturation of the precursor rRNA have been identified [8-lo] . We have found that ODC activity increases greatly when lymphocytes are cultured with PHA. This increase correlates with, and is dependent upon, the major increase in the net synthesis of rRNA.