摘要
由于NO合成不是一条闭合环路,产物C it对反应存在负反馈调节,因此,只有在增加精氨酸供应的同时清除瓜氨酸,才能促进NO合成。“瓜氨酸—一氧化氮循环”是细胞内保持精氨酸补充与瓜氨酸清除处于动态平衡的途径。在小肠、肾、脾、脑、血管平滑肌等肝外组织或细胞,虽然不存在完整的尿素循环,但存在尿素循环中的一些酶,如A S、AL,它们能将NO合成的终产物瓜氨酸重新合成精氨酸,供NO S再利用。一般来说,在生理状况下,除肝组织结构性表达丰富的A S、AL外,其他细胞只有微量的A S、AL活性,在受到免疫刺激的病理条件下,A S、AL可与iNO S共同表达,但其出现的时间、部位、程度不一。与肝实质细胞尿素循环的高效性和各酶连接的紧凑性相比,非肝细胞的“C it-NO cycle”是低效的,其低效性原因尚不可知,很可能对于大多数细胞来说,细胞外精氨酸提供了诱导性NO的大部分底物。
As nitric oxide synthesis is not a round route,the production of i-Cit will restrain the nitric oxide synthesis, which is called negative feedback. Cit elimination is necessary when increasing Arg supply. Cit-NO cycle is cellular pathway to maintain homeostasis of Arg supplementation and Cit elimination. Some of urea cycle enzymes such as argininosuccinate synthetase （AS）,argininosuccinate lyase （AL） can be expressed in kidney, spleen,lung, brain, intestines, vascular endothelial cell. Generally speaking, in physiological condition, except that the expression of AS, AL is abundant in hepatocytes and independent of lipopolysaccharide treatment,the other cells have minimum AS and AL activity. AS and AL is co-expressed with iNOS, but the time, location and expression is different. In contrast to tight channeling and resultant of urea cycle in hepatocytes,recycling of arginine to citrulline in nonhepatic cells is inefficient. The reason of inefficient is unknown. It is likely that extracellular arginine provides the majority of substrate for induced NO synthesis.
出处
《中国体育科技》
北大核心
2005年第5期130-131,140,共3页
China Sport Science and Technology
