Studies on the regulation of ornithine decarboxylase activity by the microtubules: the effect of colchicine and vinblastine

Induction of a protein inhibitor to ornithine decarboxylase by the end products of its reaction

Putrescine, the end-product of ornithine decarboxylase (ODC: L-ornithine carboxylyase, EC; 4.1.1.17) action, induces the synthesis of a protein(s), in L1210, neuroblastoma, and H-35 cells as well as in rat liver, which inhibits ODC activity. Spermidine and spermine, distal products of ODC activity, also induce the synthesis of a similar protein in H-35 cells. These ODC-inhibitors are heat-labile, trypsin-sensitive, and their induction is dependent upon protein synthesis. They have short half-lives which range from 18 to 66 min; these half-lives are similar to those of the ODC derived from the same source. They are noncompetitive inhibitors of ODC activity with an apparent molecular weight of 26,500. Each inhibitor crossreacts with the ODC's of the other cells and forms an enzyme-inhibitor complex which is stable during Sephadex chromatography; however, after treatment with ammonium sulfate, enzyme and inhibitor activities can be dissociated and recovered intact from the same column. We propose the name antizyme for proteins whose synthesis is induced by the proximal or distal products of the enzyme they inhibit.

Effects of RNase and RNA on in vitro aster assembly

RNase alters the in vitro assembly of spindle asters in homogenates of meiotically dividing surf clam (Spisula solidissima) oocytes. Some effects of RNase, such as reduced astral fiber length, appear nonenzymatic and probably result from RNase binding to tubulin. However, RNase-induced changes in the microtubule organizing center are also observed. Since other polycations can mimic RNase effects, the existence of an RNA component of the spindle organizing center remains uncertain. Effects of RNase and other polycations on astral fiber length can be prevented and reversed by the RNase inhibitor, polyguanylic acid. Polyguanylic acid can also augment astral fiber length in the absence of added RNase or other polycations. Augmentation by polyguanylic acid is favored by high ionic strength, and can be duplicated by polyuridylic acid and, with less efficiency, by polyadenylic acid. Polycytidylic acid and unfractionated yeast RNA, however, are unable to augment aster assembly. Polyguanylic acid can also augment the length of astral fibers on complete spindles isolated under polymerizing conditions. These results demonstrate that specific polyribonucleotides can alter spindle assembly in vitro. The presence of an inhibitor of microtubule assembly in Spisula oocytes, which can be inactivated by specific RNAs, is suggested.