Reciprocal Regulation of Ornithine Decarboxylase Degradation by Antizyme and Antizyme Inhibitor.

细胞内鸟氨酸脱羧酶（ODC）水平受其产物腐胺和多胺的负反馈调节。我们在前期研究中发现，多胺诱导的调节蛋白antizyme（AZ）与ODC结合，使ODC失稳，同时发现了另一种可能的调节蛋白antizyme inhibitor（AI），其大小与ODC相似，与AZ的结合亲和力高于ODC.从大鼠心脏cDNA文库中克隆的AI cDNA序列分析表明，AI是一个由448个氨基酸组成的蛋白质，与大鼠ODC高度同源（47.5%的同源性），但与大鼠ODC明显不同，缺乏含有快速降解所必需的PEST的C端区域。这一结果排除了AI是ODC的翻译后衍生物的可能性。目前正在对转染AI的强制表达效果进行检测，尚未有确切结果。从非洲爪蟾（Xenopus laevis）的肝脏中分离AZ cDNA并测定其序列。青蛙AZ cDNA与大鼠AZ cDNA有63%的同源性，并且与大鼠cDNA具有相似的结构，其翻译需要多胺诱导的核糖体移码。虽然多胺在原生动物四膜虫中引起ODC的轻度抑制，但ODC降解的速率不受多胺和ODC抑制因子的影响，无论是游离形式还是复合形式，在细胞提取物中均未检测到，这表明ODC合成的抑制单独负责多胺引起的原生动物细胞中ODC的抑制。

Cellular level of ornithine decarboxylase (ODC) is under negative feedback regulation by its products putrescine and polyamines. We elucidated previously that polyamine-induced regulatory protein, antizyme (AZ), binds with ODC,destabilizing it. We also found another possible regulatory protein, antizyme inhibitor (AI), which is of similar size with ODC and binds with AZ with higher affinity than ODC.Following results were obtained by the present study.1. Sequence analysis of AI cDNA cloned from rat heart cDNA library revealed that AI is a protein consisting of 448 amino acids, which is highly homologous (47.5 percent identity) to, but clearly distinct from rat ODC,lacking PEST containing C-terminal region essential for rapid degradation. This result excluded the possibility that AI is some post-translational derivative of ODC.2. Examination of the effect of forced expression of transfected AI is under way right now with no firm result yet.3. AZ cDNA was isolated from the liver of Xenopus laevis and its sequence determined. The frog AZ cDNA had 63 percent homology with rat AZ cDNA and had a similar structure with rat cDNA that necessitates polyamine-induced ribosomal frameshifting for its translation. Although polysmines caused mild suppression of ODC in a protozoa Tetrahymena, the rate of ODC degradation was not affected by polyamines and ODC inhibitory factor, of either free or complexed form, was not detected in the cell extract, suggesting that repression of ODC synthesis alone is responsible for the polyamine-caused suppression of ODC in the protozoan cells.