Double-stranded RNA-stimulated enzyme activities isolated from human placental extracts

A (2'-5')An synthetase activity was isolated from human placental extracts by affinity chromatography on poly(rI) . poly(rC)-agarose. The oligonucleotide (2'-5')An was identified by (1) chromatography on PEI-cellulose and DEAE-cellulose, (2) inhibition of polypeptide synthesis in lysed rabbit reticulocytes (3) competition of the binding of pppA(pA)3,3'-[32P]pCp to rabbit reticulocyte lysates, and (4) alkaline phosphatase digestion. The synthetase activity in most placental preparations is activated by natural or synthetic dsRNA. However, in a few placental synthetase preparations, dsRNA is only marginally stimulatory and only becomes effective by prior treatment of the enzyme preparations with the calcium-dependent micrococcal nuclease. This suggests that there is an endogenous placental dsRNA contaminant in the enzyme preparations. In some synthetase preparations, a second dsRNA-stimulated product, tentatively identified as the nucleotide 5'-IMP, is also observed. Because the specific AMP deaminase inhibitor coformycin (10 microM) blocks the formation of IMP from ATP and causes a quantitative accumulation of AMP, and because the formation of IMp becomes independent of dsRNA when ADP or AMP is used in place of ATP, the presence of a dsRNA-stimulated ATP phosphohydrolase (ATPase) activity in human placenta is suggested.