GTP環狀水解酵素在細胞內位置的研究

Abstract

BH4的合成主要是經由GTP，經過三種酵素的作用而來，其中由GTP將之轉變為D-erythro-7,8-dihydroneopterin triphosphate的過程是由第一種酵素作用，也是速率決定步驟，這個酵素稱之為GTP環狀水解酵素GTP cyclohydrolase 1(GCH)。GCH的基因變異為隱性的體染色體時(autosomal recessive)會造成隱性的遺傳形式(PKU)(phenylketonuria)。而HPD/DRD(hereditary progressive dystonia)/(DOPA-resonsive dystonia)患者的GCH基因缺陷則為顯性的遺傳形式(autosomal dominant), HPD/DRD患者體內的GCH的活性通常都低於20％。藉著細胞均質化與超高速離心實驗，發現cytosolic及membrane-bound 的GCH形式存在於細胞。藉著試管中轉錄與轉譯以及超高速離心實驗，我們也發現了cytosolic及 membrane－bound的GCH分佈。 很有趣的一個發現是藉著共同免疫沈澱過程GCH可以與hsp70有protein與protein間的interaction。從這些資料，我們推測GCH藉由hsp70與或著是與hsp70有接觸的protein complex而有membrane association。在未來，有更多的實驗需要來證明本篇論文所提的假說。

BH4 is synthesized from GTP by three enzymes. The conversion of GTP to D-erythro-7, 8-dihydroneopterin triphosphosphate is the first and rate-limiting step in the BH4 biosynthesis pathway and is catalyzed by GTP cyclohydrolase (GCH). Mutations in GCH gene were found to cause both GCH deficiency with autosomal recessive trait and hereditary progressive dystonia (HPD)/DOPA-responsive dystonia (DRD) with autosomal dominant trait. When GCH activity is decreased to less than 20% of the normal value, the activity of tyrosine hydroxylase in the nigrostriatal dopaminergic neurons may be first decrease resulting in tyrosine hydroxylase activity and dopamine level, and in the symptoms of HPD/DRD. By homogenization coupled with ultracentrifugation experiment, cytosolic and membrane-bound forms of GCH were found in the cell. By in vitro transcription/translation coupled with ultracentrifugation experiment, cytosolic and membrane bound forms of GCH were also detected. Interestingly, GCH was found to interact with hsp70 by co-immunoprecipitation experiment. These data indicate that the membrane association of GCH would be mediated by hsp70 or hsp70 associated protein complex. however, more experiments should be done.