Metabolically Responsive Contrast or Imaging Agents for NMR or ESR Imaging: Metabolism of Nitroxides in Cells

107 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.The use of nitroxide spin labels in complex biological systems as biophysical tools to study molecular dynamics of membranes and cells and as contrast or imaging agents for NMR or ESR imaging and in vivo spectroscopy has led to a need to understand thoroughly the metabolism of nitroxides in cells. A fundamental investigation of the metabolism of nitroxide in cells is presented in this thesis.Nitroxides can be reduced in cells. The products of cellular reduction of nitroxides are the corresponding hydroxylamines. The selective action of inhibitors, and thermal and chemical inactivation demonstrate that the reduction of nitroxides in cells is an enzymatic or enzyme-mediated process. The kinetics of reduction of doxyl stearates are affected by the position of the doxyl moiety along the stearic acid chain. The rate limiting step for the reduction of the doxyl moiety deep in the membrane is the diffusion of reducing equivalents within or into the membrane. The reduction of doxyl stearates in cells is inhibited by rotenone but not antimycin A, cyanide, propyl gallate or SKF-525A. It appears that the reduction of doxyl stearates takes place at the level of the ubiquinone in the respiratory chain of mitochondria in these cells.Hydroxylamines can be oxidized to nitroxides by cells in the presence of oxygen. Only lipophilic hydroxylamines are oxidized rapidly. Aqueous soluble hydroxylamines are not oxidized by cells, except for a slight oxidation of some pyrrolidine derivatives. The latter is due to autoxidation. The enzyme-mediated oxidation occurs within the membrane, with cytochrome c oxidase apparently responsible for this oxidation.The metabolism of nitroxides in cells, including reduction of nitroxides and oxidation of hydroxylamines, is greatly affected by oxygen. In order to separate effects due directly to oxygen from those due to the redox state of enzymes on the rates of metabolism of nitroxides, the cells were studied under conditions in which each of these variables could be changed independently. The results suggest that oxygen affects the rate of reduction of nitroxides by changing the redox state of the enzymes in the respiratory chain in cells.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD