Nonenzymatic bioreduction in rat liver and kidney of nitroxyl spin labels, potential contrast agents in magnetic resonance imaging.

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)