Nitrite (NO 2 -), being a product of metabolism of both nitric oxide (NO • ) and nitrate (NO 3 -), can accumulate in tissues and regenerate NO • by several mechanisms. The effect of NO 2 -on ischemia/ reperfusion injury was also reported. Nevertheless, the mechanisms of intracellular NO 2 -accumulation are poorly understood. We suggested significant role of nitrite penetration through biological membranes in the form of undissociated nitrous acid (HNO 2 ). This hypothesis has been tested using large unilamellar phosphatidylcholine liposomes and several spectroscopic techniques. HNO 2 transport across the phospholipid bilayer of liposomes facilitates proton transfer resulting in intraliposomal acidification, which was measured using pH-sensitive probes. NO 2 --mediated intraliposomal acidification was confirmed by EPR spectroscopy using membrane-impermeable pHsensitive nitroxide, 2,2,5,5-tetramethyl-1-oxyl-2,5-dihydro-1H-imidazol-3-ium-4-yl)-aminomethanesulfonic acid (pK 5.25), and by 31 P-NMR spectroscopy using inorganic phosphate (pK 6.9). Nitrite accumulates inside liposomes in concentration exceeding its concentration in the bulk solution, when initial transmembrane pH gradient (alkaline inside) is applied. Intraliposomal accumulation of NO 2 -was observed by direct measurement using chemiluminescence technique. Perfusion of isolated rat hearts with buffer containing 4 μM NO 2 -was performed. The nitrite concentrations in the effluent and in the tissue, measured after 1 minute perfusion were close, supporting fast penetration of the nitrite through the tissue. Measurements of the nitrate/nitrate showed that total concentration of NO x in myocardium increased from initial 7.8 μM to 24.7 μM after nitrite perfusion. Physiological significance of passive transmembrane transport of NO 2 -and its coupling with intraliposomal acidification are discussed.

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Longitudinal relaxation (T 1) measurements for all lines (N(CH3)2, N(CH3), (C(CH3), phenyl) in the aminopyrine 1H‐NMR spectrum were used to study the interaction of aminopyrine with purified microsomal cytochrome P‐450 from livers of phenobarbital‐treated rats. The paramagnetic contribution to the observed t 1 −1 values was determined from its dependence on aminopyrine concentration. The Solomon‐Bloembergen equation was used to calculate between Fe3+ and aminopyrine distances in the enzyme‐substrate complex. For all protons these distances are about 8 Å.

Longitudinal relaxation (T1) measurements for all lines (α‐CH, β‐CH, O‐CH3) in the 4‐methoxypyridine 1H‐NMR spectrum were used to study the interaction of 4‐methoxypyridine with purified microsomal cytochrome P‐450 from livers of phenobarbital‐treated rats. The paramagnetic contribution to the observed T1 −1 value was determined from its dependence on 4‐methoxypyridine concentration. In the P‐450‐4‐methoxypyridine complex the latter is oriented so that the nitrogen of pyridine is directed towards the Fe3+.