Hamilton J W, Bement W J, Sinclair P R, Sinclair J F, Alcedo J A, Wetterhahn K E
Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755.
Arch Biochem Biophys. 1991 Sep;289(2):387-92. doi: 10.1016/0003-9861(91)90428-l.
Hepatic 5-aminolevulinate (ALA) synthase, the first and rate-limiting enzyme in the heme biosynthetic pathway, is known to be feedback repressed by the end product of the pathway, heme. We investigated whether heme regulates ALA synthase mRNA expression transcriptionally or post-transcriptionally in primary cultures of chick embryo hepatocytes. 2-Propyl-2-isopropylacetamide increased the rate of transcription of the ALA synthase gene, whereas heme or an inhibitor of heme biosynthesis, desferrioximine, had no effect on the drug-induced transcription rate. Heme decreased the half-life of ALA synthase mRNA from approximately 3.5 h to 1.2 as recently reported by Drew and Ades (1989, Biochem. Biophys. Res. Commun. 162, 102-107). We also found that the heme-mediated decrease in mRNA stability was prevented by cycloheximide treatment, suggesting that the heme effect was mediated by a labile protein. These results support a model for hepatic ALA synthase regulation in which inducing drugs directly stimulate ALA synthase gene transcription, whereas heme regulates ALA synthase expression post-transcriptionally by modulating mRNA stability as well as by blocking translocation of ALA synthase enzyme into the mitochondrion.
肝5-氨基酮戊酸（ALA）合酶是血红素生物合成途径中的首个限速酶，已知该途径的终产物血红素可对其进行反馈抑制。我们研究了在鸡胚肝细胞原代培养物中，血红素是在转录水平还是转录后水平调节ALA合酶mRNA的表达。2-丙基-2-异丙基乙酰胺增加了ALA合酶基因的转录速率，而血红素或血红素生物合成抑制剂去铁胺对药物诱导的转录速率没有影响。正如德鲁和阿德斯最近报道的那样（1989年，《生物化学与生物物理研究通讯》162卷，第102 - 107页），血红素将ALA合酶mRNA的半衰期从约3.5小时缩短至1.2小时。我们还发现，环己酰亚胺处理可防止血红素介导的mRNA稳定性降低，这表明血红素的作用是由一种不稳定的蛋白质介导的。这些结果支持了一种肝ALA合酶调节模型，即诱导药物直接刺激ALA合酶基因转录，而血红素通过调节mRNA稳定性以及阻止ALA合酶酶转运到线粒体中，在转录后水平调节ALA合酶的表达。
