The Effect of Ammonium Ion on the Induction of (2'-5')-(A) n Synthetase and (2'-5')-(A) n -Dependent Endoribonuclease Activities by Interferon in Mouse BALB/C 3T3 Cells

Abstract

Addition of ammonium chloride to interferon (IFN)-treated mouse L cells has been reported to reduce the antiviral state. Because (2'-5')-(A) _n synthetase and (2'-5')-(A) _n -dependent endoribonuclease (RNase L) activities have been implicated in the establishment of the antiviral state, we wish to ascertain if ammonium ions will also impair the IFN-elicited induction of these two enzymatic activities. To test this possibility, cell-free extracts were prepared from mouse BALB/C 3T3 cells and assayed for (2'-5')-(A) _n synthetase and RNase L activities. Results of these studies show that when cells were incubated with IFN (100 or 500 U/ml) and ammonium chloride (20–40 mM) for 20 h, the induction of (2'-5')-(A) _n synthetase by IFN is significantly suppressed. Based on binding to the specific radioactive (2'-5')-(A) _n analog, (2'-5')p₃A₄,3'-[³²P]pCp, on cross-linking to the periodate-oxidized (2'-5')p₃A₄,3'-[³²P]pC, or on the (2'-5')-(A) _n -enhanced degradation of [³H]-polyadenylated RNA, IFN (10–250 U/ml) was shown to cause a two- to sevenfold increase in RNase L activity. The induction of RNase L by IFN was blocked by simultaneous addition of ammonium chloride. However, 20 mM or 40 mM ammonium chloride did not affect the antiviral state in BALB/C 3T3 cells (based on a plaque reduction assay with 10–500 U/ml IFN). These results suggest that the establishment of the antiviral state is not necessarily coordinated with changes in the synthetase and the RNase L activities.