TOXICITY OF AN ANTITUMOR RIBONUCLEASE TO PURKINJE NEURONS

Purkinje cell toxicity is one of the characteristic features of the Go rdon phenomenon, a syndrome manifested by ataxia, muscular rigidity, p aralysis, and tremor that may lead to death (Gordon, 1933). Two member s of the RNase superfamily found in humans, EDN (eosinophil-derived ne urotoxin) and ECP (eosinophil cationic protein), cause the Gordon phen omenon when injected intraventricularly into guinea pigs or rabbits. W e have found that another member of the RNase superfamily, an antitumo r protein called onconase, isolated from Rana pipiens oocytes and earl y embryos, will also cause the Gordon phenomenon when injected into th e cerebrospinal fluid of guinea pigs at a dose similar to that of EDN (LD(50), 3-4 mu g). Neurologic abnormalities of onconase-treated anima ls were indistinguishable from those of EDN-treated animals, and histo logy showed dramatic Purkinje cell loss in the brains of onconase-trea ted animals. The neurotoxic activity of onconase correlates with ribon uclease activity. Onconase modified by iodoacetic acid to eliminate 70 % and 98% of the ribonuclease activity of the native enzyme displays a similar decrease in ability to cause the Gordon phenomenon. In contra st, the homologous bovine pancreatic RNase A injected intraventricular ly at a dose 5000 times greater than the LD,, dose of EDN or onconase is not toxic and does not cause the Gordon phenomenon. A comparison of the RNase activities of EDN, onconase, and bovine pancreatic RNase A using three pancreatic RNA substrates demonstrates that onconase is or ders of magnitude less active enzymatically than EDN and RNase A. Thus , another member of the RNase superfamily in addition to EDN and ECP c an cause the Gordon phenomenon. Ribonuclease activity of onconase appe ars essential for onconase-mediated neurotoxicity. However, substantia l differences in neurotoxicity observed among some homologous ribonucl eases cannot be due to their different enzymatic activities; other fea tures of the enzymes are considered.