Eriksson U G, Brasch R C, Tozer T N
Drug Metab Dispos. 1987 Mar-Apr;15(2):155-60.
Paramagnetic nitroxyl spin labels have potential clinical utility as contrast agents in proton magnetic resonance imaging. Reduction of the nitroxyl moiety in vivo results in the formation of the diamagnetic hydroxylamine, which lacks contrast-enhancing activity. Bioreduction is therefore an important determinant of the imaging behavior of these agents. Both enzymatic and nonenzymatic reduction mechanisms have been suggested for nitroxyl spin labels. This study examines the nonenzymatic mechanisms in rat liver and kidney, mammalian tissues that demonstrate high reducing activity. Protein-free preparations, obtained by heat precipitation or ultrafiltration of rat liver and kidney homogenates, were used to test piperidine and pyrrolidine nitroxyl spin-label derivatives, for which imaging properties and bioreduction had previously been examined. For the piperidine derivative, the initial reduction rates in ultrafiltrates and supernatant fluids were 25-60% of those in whole liver and kidney homogenates. However, the pyrrolidine derivative was reduced at rates much slower than those in whole tissue homogenates. The reduction in whole tissue homogenates was NADPH-dependent, while reduction in ultrafiltrates was unaffected by the addition of NADPH. Preincubation of the ultrafiltrates and supernatant fluids with ascorbic acid oxidase caused almost complete inhibition of the reduction. The reduction rates of these nitroxyl derivatives were determined in ascorbic acid solution; second order rate constants were 0.45 +/- 0.04 and 0.0042 +/- 0.001 mM-1 min-1 for the piperidine and pyrrolidine derivatives, respectively. The concentrations of ascorbic acid in the supernatant fluids and ultrafiltrates of rat liver and kidney were then predicted from the observed reduction rates and found to be virtually identical with those from spectrophotometric determinations.(ABSTRACT TRUNCATED AT 250 WORDS)
顺磁性硝酰自旋标记物作为质子磁共振成像中的造影剂具有潜在的临床应用价值。体内硝酰部分的还原会导致形成缺乏造影增强活性的抗磁性羟胺。因此，生物还原是这些造影剂成像行为的一个重要决定因素。对于硝酰自旋标记物，已经提出了酶促和非酶促还原机制。本研究考察了大鼠肝脏和肾脏中的非酶促机制，这两种哺乳动物组织表现出较高的还原活性。通过对大鼠肝脏和肾脏匀浆进行热沉淀或超滤获得的无蛋白制剂，用于测试哌啶和吡咯烷硝酰自旋标记衍生物，此前已对其成像特性和生物还原进行过研究。对于哌啶衍生物，超滤液和上清液中的初始还原速率是全肝和全肾匀浆中还原速率的25% - 60%。然而，吡咯烷衍生物的还原速率比全组织匀浆中的还原速率慢得多。全组织匀浆中的还原是NADPH依赖性的，而超滤液中的还原不受添加NADPH的影响。将超滤液和上清液与抗坏血酸氧化酶预孵育几乎完全抑制了还原。在抗坏血酸溶液中测定了这些硝酰衍生物的还原速率；哌啶和吡咯烷衍生物的二级速率常数分别为0.45±0.04和0.0042±0.001 mM-1 min-1。然后根据观察到的还原速率预测大鼠肝脏和肾脏上清液和超滤液中抗坏血酸的浓度，发现其与分光光度法测定的浓度几乎相同。（摘要截短于250字）
