DIRECT MEASUREMENT OF THE ACCUMULATION AND MITOCHONDRIAL CONVERSION OF NITRIC-OXIDE WITHIN CHINESE-HAMSTER OVARY CELLS USING AN INTRACELLULAR ELECTRON-PARAMAGNETIC-RESONANCE TECHNIQUE

We have developed an electron paramagnetic resonance (EPR) method for the nondestructive detection and quantification of intracellular NO in real time. Based upon this technique, we have obtained evidence for t he metabolism of this bioregulatory molecule by mitochondria. Line-bro adening of the EPR signal of a coal derivative, fusinite, was calibrat ed as a function of NO concentration in aqueous solution. The methodol ogy was validated using two compounds which release NO in a controlled and predictable manner with first-order rate constants k(1) = 5.0 . 1 0(-3) s(-1) and k(1) = 3.4 . 10(-4) s(-1) (35 degrees C). Fusinite was internalized in Chinese hamster ovary cells (CHO) by phagocytosis, af ter which the cells were allowed to consume the available O-2, produci ng an hypoxic environment. The NO released from one of the NO donors, added to the culture fluid at an initial concentration of 50 mu M, was directly measured in the intracellular environment as line-broadening of the fusinite EPR signal. The linewidth diminished with time, indic ating that NO was being converted to a non-paramagnetic species by the cells with an apparent zero-order rate constant of 5 . 10(8) NO molec ules cell(-1) min(-1) (20 degrees C). Addition of cyanide to the cultu re medium (5 mM final concentration) inhibited this disappearance of N O. NO also was converted in the presence of isolated mitochondria in t he absence of oxygen. These observations suggest that under hypoxic co nditions, there exists in CHO cells a metabolic pathway for the conver sion of NO to diamagnetic species, which involves interactions with mi tochondria.